Apply root element margins and propagate body background to the canvas

[nicoburns]

Summary

Fixes WPT css/CSS2/margin-padding-clear/margin-collapse-020.xht and margin-collapse-021.xht.

Two related root/body issues:

1. Root element margins — taffy::compute_root_layout resolves the root element's margins but always positions its border box at the origin. resolve_layout now offsets the root's location by its resolved margins after root layout (RTL-aware for the horizontal axis); per CSS2 §8.3.1 the root's margins never collapse with descendants.

2. Body background propagation (CSS Backgrounds §2.11.2) — when the root element's background is entirely transparent/none, the body's background propagates to the canvas. Previously only the body's background color was used as the canvas color while the body still painted its background at its own position; with root margins applied this left the canvas mispositioned relative to the test's expectation. BlitzDomPainter now resolves a canvas_bg_body_id up front; that body's background (color and images) is painted for the canvas with its positioning area determined as if it were painted for the root element, and is skipped during the body's own painting:

paint_scene:
  fill viewport with body's background-color
  draw body's background with root element's layout/transform
render_element:
  if canvas_bg_body_id != Some(node_id) { cx.draw_background(scene) }

background_is_none checks both background-color: transparent and all background-image: none, so a root with only a background image (as in margin-collapse-020) correctly prevents propagation.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/19d83a7d94fb4e43a489abaa566115ba
Requested by: @nicoburns

WPT results

16 newly passing, 7 newly failing (net +9).

Full diff (23 changed tests)

- Pass => Fail css/CSS2/backgrounds/background-position-002.xht
+ Fail => Pass css/CSS2/backgrounds/background-root-004.xht
+ Fail => Pass css/CSS2/backgrounds/background-root-005.xht
+ Fail => Pass css/CSS2/backgrounds/background-root-006.xht
+ Fail => Pass css/CSS2/backgrounds/background-root-011.xht
+ Fail => Pass css/CSS2/backgrounds/background-root-012a.xht
+ Fail => Pass css/CSS2/backgrounds/background-root-012b.xht
+ Fail => Pass css/CSS2/backgrounds/background-root-013a.xht
+ Fail => Pass css/CSS2/backgrounds/background-root-013b.xht
+ Fail => Pass css/CSS2/backgrounds/background-root-014a.xht
+ Fail => Pass css/CSS2/backgrounds/background-root-014b.xht
+ Fail => Pass css/CSS2/backgrounds/background-root-015.xht
+ Fail => Pass css/CSS2/backgrounds/background-root-017.xht
+ Fail => Pass css/CSS2/backgrounds/background-root-020.xht
- Pass => Fail css/CSS2/floats-clear/margin-collapse-clear-017.xht
+ Fail => Pass css/CSS2/margin-padding-clear/margin-collapse-020.xht
+ Fail => Pass css/CSS2/margin-padding-clear/margin-collapse-021.xht
+ Fail => Pass css/CSS2/normal-flow/root-box-001.xht
- Pass => Fail css/css-backgrounds/box-shadow-body.html
- Pass => Fail css/css-image-animation/image-animation-body-background-no-propagation-paused.html
- Pass => Fail css/css-images/linear-gradient-body-sibling-index.html
- Pass => Fail css/css-transforms/perspective-split-by-zero-w.html
- Pass => Fail css/css-transforms/transform-background-007.html

_{Generated by the WPT workflow.}

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