Fix RenderBackgroundSystem::drawImage background-repeat logic for negative repeat regions

[Copilot]

The background-repeat implementation was incorrectly handling image tiles in negative directions when background-origin created an offset from the border-box. This caused visible gaps in repeated background images.

Problem

When background-repeat was set to repeat-x or repeat-y and background-origin was set to content-box or padding-box, the tiling logic would only generate tiles from the origin position outward, leaving regions before the origin uncovered.

For example:

• Border-box: x=0 to x=300
• Content-box: x=100 to x=300 (100px offset due to border + padding)
• With background-origin: content-box and repeat-x, tiles would only appear from x=100 onwards
• The region from x=0 to x=100 would remain uncovered, despite being within the paintable area

Root Cause

The drawImage function was using the positioningArea (calculated from background-origin) for both:

1. Initial image positioning ✅ (correct usage)
2. Tiling loop bounds ❌ (should use background-clip area)

This violated the CSS specification where background-origin should only affect the initial position offset, not the extent of repeated tiles.

Solution

• Added overloaded drawImage function accepting separate positioning and repeatable areas
• Modified tiling loops to use repeatableArea bounds instead of positioningArea bounds
• Updated caller to pass background-clip area as repeatable area and background-origin area as positioning area
• Ensured all repeat modes (repeat, repeat-x, repeat-y) use the correct repeatable bounds

Test Results

// Before fix: tiles only from x=100 to x=300 (gap from x=0 to x=100)
// After fix: tiles cover full x=0 to x=300 area
float oldStartX = 100.0f; // Only covered positioning area
float newStartX = 0.0f;   // Covers full repeatable area
int tilesInNegativeRegion = 2; // Now properly tiles the "negative" region

The fix ensures background images properly tile across the entire visible area while respecting the positioning offset specified by background-origin.

Fixes #279.

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