fix(layers): keep IconLayer and TextLayer visible on globe projections

[wldalx]

Closes #9777

Background

Globe back-face culling incorrectly hides icons, text glyphs and text backgrounds because their shaders flip Y, reversing triangle winding.

Change List

• Correct IconLayer winding for icons and text glyphs
• Correct TextBackgroundLayer GLSL/WGSL winding, preserving content-box backgrounds
• Add text-layer-globe golden image for background visibility
• Release test WebGL contexts to prevent context exhaustion
• Unit tests: MapLibre projections, overlay modes, billboarding and picking
• Render tests: globe text and backgrounds on WebGL/WebGPU

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Note

Medium Risk
Shader and geometry winding changes affect all IconLayer and text-background rendering paths; behavior is intentionally scoped but could alter edge-case appearance outside globe view.

Overview
Fixes icons and text backgrounds disappearing on globe projections when back-face culling is enabled. Vertex shaders flip Y, which had reversed triangle winding so quads were treated as back-facing.

IconLayer reorders the shared quad positions triangle-strip so winding stays front-facing after the Y flip. TextBackgroundLayer (GLSL and WGSL) derives a corner UV that mirrors Y only for natural-bounds paths (instanceClipRect.w < 0), keeping content-box backgrounds unchanged while fixing winding for the flip-Y path.

Adds MapLibre integration tests (mercator and globe, interleaved overlays, billboard on/off) that assert label pixels, text background visibility, and picking; non-interleaved runs explicitly release WebGL contexts to avoid exhausting browser limits.

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[greptile-apps]

Confidence Score: 5/5

[Medium impact] The PR appears safe to merge; no actionable issues were found.

Summary

This PR corrects triangle winding so globe back-face culling keeps icons, text glyphs, and text backgrounds visible.

• Reorders the IconLayer quad without changing its texture mapping.
• Updates both background shaders while preserving content-box bounds.
• Adds MapLibre checks for projection, overlay mode, billboarding, visibility, and picking.
• Releases separate WebGL contexts after each overlay test.

The current head does not descend from the previous reviewed commit, so the full PR diff was reviewed. No previous review threads were supplied. Tests and visual captures were not run; dependencies are not installed.

_{Reviews (3) · Last reviewed commit: "fix(layers): keep TextLayer backgrounds ..." · Reviewed by Greptile}

[chrisgervang]

Thanks for working on this! Closing as a duplicate of #10750, which addresses #9777 along with #9592 and #9554. It includes the same IconLayer winding change, fixes MultiIconLayer glyphs and text backgrounds, and also hides billboarded icons, text and points on the far side of the globe. Without that last part, they would render through the globe once culling no longer removes them.