fix: align SDK7 UiBackground nine-slice with Unity behavior

[kuruk-mm]

Closes #2060

Summary

Brings the Godot port of PBUiBackground (component 1053) closer to the Unity explorer's DCLImage semantics.

The most user-visible change: nine-slice panels (e.g. HUD panel_v2.png with textureSlices: 0.18) no longer tile their edge strips and inner area — the wood frame edges and cream interior now stretch smoothly, matching Unity UI Toolkit and ending the long-standing TODO: should be TILE or STRETCH? in dcl_ui_background.rs.

What changed in lib/src/godot_classes/dcl_ui_background.rs

• Nine-slice axis stretch: AxisStretchMode::TILE_FIT → STRETCH on both axes. Matches Unity's default sliced rendering.
• Slice clamping: opposing slices (left+right, top+bottom) are clamped to 1.0 before being applied as patch margins, mirroring Unity's DCLImage.AdjustSlices().
• UV index convention: protobuf UVs are now interpreted as BL=uvs[0..2], TL=uvs[2..4], TR=uvs[4..6], BR=uvs[6..8] (Unity's Extensions.ToDCLUVs). This affects:
  • has_custom_uvs() axis-alignment check
  • the axis-aligned region_rect computation in STRETCH mode (also fixes a latent bug where the old 1.0 - max/min formula could produce a negative region_rect height for valid inputs)
  • the parameter packing fed to dcl_ui_background_uv.gdshader (the shader itself is unchanged; we just feed it (TL, BL) / (BR, TR) from the new index order)

What is not changed

• The custom-UV shader's interpolation/Y-flip math is left alone. Unity's stretch mode uses custom mesh generation, while Godot uses a fragment shader; aligning the rotated/skewed UV path beyond index ordering would be a separate change.
• Tracing/debug logs are kept as-is.

Test plan

• Open a scene that uses Panel (uiBackground.textureMode = 'nine-slices', textureSlices: 0.18 per side, panel_v2.png) and verify the wood-frame edges no longer show repeating tiles
• Verify panels at very small sizes (parent < texture) still render with the corners intact and no clipping regression
• Spot-check other 9-slice users in the HUD (chat panel, dialog, settings) for stretching artifacts
• Verify STRETCH mode (textureMode: 'stretch') with default UVs still renders the full texture
• Verify a stretch background with axis-aligned custom UVs (e.g. a sub-region of a sprite atlas) still picks the correct region
• (optional) Verify a stretch background with rotated UVs still renders something reasonable via the shader path

[github-actions]

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🔄 Updated: 2026-05-09 16:57:20 UTC

[github-actions]

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🍏 iOS Build Complete

✅ Status: Success

🔗 iOS Workflow: View build

📦 Artifacts: godot-mobile-deploy-pipeline

📍 Branch: PR-2006-fix/ui-background-nine-slice-unity-parity

🔄 Completed: 2026-05-11 00:41:30 UTC