Refactor background layer tiling into per-axis helpers

[nicoburns]

Summary

Preparatory refactor for background-attachment: collapses the duplicated per-axis repeat/tiling math in draw_raster_image_layer and draw_gradient_layer into two pure per-axis helpers returning a uniform plan:

struct AxisTiling { translate: f64, rect_len: f64, count: u32, gap: f64 }

fn raster_axis_tiling(repeat, origin_start, origin_len, bg_pos, tile_len, image_len, ratio) -> AxisTiling
fn gradient_axis_tiling(repeat, origin_start, origin_len, clip_start, clip_len, bg_pos, tile_len) -> AxisTiling

Both draw fns now reduce to: compute x/y plans, build one transform from x.translate/y.translate, and run a single count × count fill loop. The layer geometry (positioning-area rect + translation) is now computed in exactly one place per path, so background-attachment: fixed/local can later be implemented by adjusting the axis inputs at a single site.

Semantics preserved, with two intentional deviations:

• The gradient Repeat/Round special cases for the three (clip ⊃ origin) box pairs are generalized geometrically: tiling starts from the clip box edge whenever clip_start < origin_start, replacing the exhaustive (BorderBox, PaddingBox)-style tuple matches (equivalent for all existing combinations).
• Fixes copy-paste bugs in the old gradient y-axis code, which used bg_pos_x instead of bg_pos_y in three extend(...) calls (affecting vertically-repeating gradients with a non-zero vertical background position).

Verified no regressions: just wpt css/css-backgrounds produces byte-identical per-test results before/after (250/705 passing, zero status changes).

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/adffb5f273074c819eda1794acd87032
Requested by: @nicoburns

WPT results

No changes in test results compared to main.

_{Generated by the WPT workflow.}

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