Re-resolve viewport-dependent lengths for viewBox-less SVG against the used size

[nicoburns]

Summary

Stacked on #634. An SVG with a viewBox scales to its CSS box (object-fit contain, unchanged). An SVG without one cannot be scaled: its percentage lengths must instead re-resolve against the element's used size.

SvgImageData now retains the SVG source, and tree_for_viewport(w, h) re-parses it with the root width/height attributes replaced by the used content-box size (cached per size; usvg has no API to override the root viewport after parsing, hence set_root_svg_size_attrs rewriting the root tag):

// draw_svg
let svg = svg.tree_for_viewport(zoom.unzoom(width / scale), zoom.unzoom(height / scale));

The viewport is re-resolved in unzoomed CSS px so the existing object-fit transform applies zoom/HiDPI scaling to 1:1 content (keeps css/css-viewport/zoom/svg-path.html passing). Fixes e.g. css/CSS2/normal-flow/block-replaced-height-006.xht and css/css-transforms/{scale,rotate,skewX,skewY}/svg-*, where viewBox-less SVG content was previously object-fit scaled against usvg's fallback size.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/51256d0b440341bd874c6dc59e138478
Requested by: @nicoburns

WPT results

79 newly passing, 5 newly failing (net +74).

Full diff (84 changed tests)

+ Fail => Pass css/CSS2/backgrounds/background-001.xht
+ Fail => Pass css/CSS2/backgrounds/background-002.xht
+ Fail => Pass css/CSS2/backgrounds/background-003.xht
+ Fail => Pass css/CSS2/backgrounds/background-006.xht
+ Fail => Pass css/CSS2/backgrounds/background-007.xht
+ Fail => Pass css/CSS2/backgrounds/background-008.xht
+ Fail => Pass css/CSS2/backgrounds/background-009.xht
+ Fail => Pass css/CSS2/backgrounds/background-010.xht
+ Fail => Pass css/CSS2/backgrounds/background-014.xht
+ Fail => Pass css/CSS2/backgrounds/background-018.xht
+ Fail => Pass css/CSS2/backgrounds/background-022.xht
+ Fail => Pass css/CSS2/backgrounds/background-087.xht
+ Fail => Pass css/CSS2/backgrounds/background-182.xht
+ Fail => Pass css/CSS2/backgrounds/background-328.xht
+ Fail => Pass css/CSS2/backgrounds/background-329.xht
- Pass => Fail css/CSS2/backgrounds/background-attachment-applies-to-007.xht
- Pass => Fail css/CSS2/backgrounds/background-attachment-applies-to-009.xht
- Pass => Fail css/CSS2/backgrounds/background-attachment-applies-to-012.xht
- Pass => Fail css/CSS2/backgrounds/background-attachment-applies-to-013.xht
- Pass => Fail css/CSS2/backgrounds/background-attachment-applies-to-014.xht
+ Fail => Pass css/CSS2/backgrounds/background-image-cover-002.xht
+ Fail => Pass css/CSS2/backgrounds/background-image-cover-004.xht
+ Fail => Pass css/CSS2/backgrounds/background-image-cover-attachment-001.xht
+ Fail => Pass css/CSS2/backgrounds/background-image-transparency-001.xht
+ Fail => Pass css/CSS2/backgrounds/background-repeat-001.xht
+ Fail => Pass css/CSS2/backgrounds/background-repeat-002.xht
+ Fail => Pass css/CSS2/backgrounds/background-repeat-003.xht
+ Fail => Pass css/CSS2/backgrounds/background-repeat-005.xht
+ Fail => Pass css/CSS2/borders/border-bottom-width-003.xht
+ Fail => Pass css/CSS2/borders/border-bottom-width-025.xht
+ Fail => Pass css/CSS2/borders/border-bottom-width-058.xht
+ Fail => Pass css/CSS2/borders/border-bottom-width-069.xht
+ Fail => Pass css/CSS2/borders/border-bottom-width-080.xht
+ Fail => Pass css/CSS2/borders/border-top-width-003.xht
+ Fail => Pass css/CSS2/borders/border-top-width-025.xht
+ Fail => Pass css/CSS2/borders/border-top-width-058.xht
+ Fail => Pass css/CSS2/borders/border-top-width-069.xht
+ Fail => Pass css/CSS2/borders/border-top-width-080.xht
+ Fail => Pass css/CSS2/borders/border-width-shorthand-001.xht
+ Fail => Pass css/CSS2/borders/border-width-shorthand-002.xht
+ Fail => Pass css/CSS2/borders/border-width-shorthand-003.xht
+ Fail => Pass css/CSS2/borders/border-width-shorthand-004.xht
+ Fail => Pass css/CSS2/floats-clear/float-replaced-width-011.xht
+ Fail => Pass css/CSS2/inline-svg-100-percent-in-body.html
+ Fail => Pass css/CSS2/inline-svg-intrinsic-size-100-percent-2.html
+ Fail => Pass css/CSS2/normal-flow/block-replaced-width-006.xht
+ Fail => Pass css/CSS2/normal-flow/inline-block-replaced-width-006.xht
+ Fail => Pass css/CSS2/normal-flow/inline-replaced-width-006.xht
+ Fail => Pass css/CSS2/positioning/absolute-replaced-width-006.xht
+ Fail => Pass css/CSS2/positioning/absolute-replaced-width-013.xht
+ Fail => Pass css/CSS2/positioning/absolute-replaced-width-020.xht
+ Fail => Pass css/CSS2/positioning/absolute-replaced-width-027.xht
+ Fail => Pass css/CSS2/positioning/absolute-replaced-width-034.xht
+ Fail => Pass css/CSS2/positioning/absolute-replaced-width-037.xht
+ Fail => Pass css/CSS2/positioning/absolute-replaced-width-051.xht
+ Fail => Pass css/CSS2/positioning/absolute-replaced-width-069.xht
+ Fail => Pass css/CSS2/positioning/absolute-replaced-width-076.xht
+ Fail => Pass css/css-borders/corner-shape/corner-shape-svg-border.html
+ Fail => Pass css/css-images/object-position-svg-001i.html
+ Fail => Pass css/css-images/object-position-svg-002i.html
+ Fail => Pass css/css-transforms/css-skew-002.html
+ Fail => Pass css/css-transforms/gradientTransform/svg-gradientTransform-combination-001.html
+ Fail => Pass css/css-transforms/group/svg-transform-group-008.html
+ Fail => Pass css/css-transforms/group/svg-transform-group-009.html
+ Fail => Pass css/css-transforms/group/svg-transform-group-011.html
+ Fail => Pass css/css-transforms/group/svg-transform-nested-008.html
+ Fail => Pass css/css-transforms/group/svg-transform-nested-013.html
+ Fail => Pass css/css-transforms/group/svg-transform-nested-014.html
+ Fail => Pass css/css-transforms/group/svg-transform-nested-018.html
+ Fail => Pass css/css-transforms/group/svg-transform-nested-021.html
+ Fail => Pass css/css-transforms/group/svg-transform-nested-029.html
+ Fail => Pass css/css-transforms/scale/svg-scale-006.html
+ Fail => Pass css/css-transforms/scale/svg-scale-007.html
+ Fail => Pass css/css-transforms/skewX/svg-skewx-001.html
+ Fail => Pass css/css-transforms/skewX/svg-skewx-006.html
+ Fail => Pass css/css-transforms/skewX/svg-skewx-011.html
+ Fail => Pass css/css-transforms/skewX/svg-skewx-016.html
+ Fail => Pass css/css-transforms/skewX/svg-skewx-021.html
+ Fail => Pass css/css-transforms/skewY/svg-skewy-001.html
+ Fail => Pass css/css-transforms/skewY/svg-skewy-006.html
+ Fail => Pass css/css-transforms/skewY/svg-skewy-011.html
+ Fail => Pass css/css-transforms/skewY/svg-skewy-016.html
+ Fail => Pass css/css-transforms/skewY/svg-skewy-021.html
+ Fail => Pass css/css-transforms/translate/svg-translate-with-units.html

_{Generated by the WPT workflow.}

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