Implements full pipeline support for background-position (shorthand and
longhands) and background-repeat so that CSS sprite techniques like
`background: url(sprite.png) 0 -261px repeat-x` render correctly.
New shared types BackgroundPositionValue and BackgroundRepeat flow through
CSS parsing, style computation, layout, display list, and rasterization.
Background images now render at intrinsic dimensions with tiling, and
position values are resolved at rasterization time using the CSS spec
formula for percentage positioning.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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