feat(layers): port TextLayer glyph rendering to WebGPU

[ibgreen-openai]

Goal

Port the core TextLayer glyph rendering path to WebGPU as a small, reviewable extraction from #10450.

Changes

• Add a WGSL shader for MultiIconLayer, the glyph-rendering sublayer used by TextLayer.
• Select the WGSL source only when the active device is WebGPU; WebGL continues to use the existing GLSL shader path.
• Pack the TextLayer-specific row index and clip-rect attributes into a WebGPU buffer group.
• Add WebGL2/WebGPU tabs to the website TextLayer example and pass the selected device through to DeckGL.
• Skip CollisionFilterExtension only for WebGPU because that extension is not ported yet; WebGL keeps the existing collision-filter behavior.

Website examples

Layer / feature	Example	WebGPU status	Notes
TextLayer glyph rendering	World Cities by Population	✅ Works	CollisionFilterExtension is skipped on WebGPU only.

TextBackgroundLayer and the clipping/treemap example are intentionally left out of this PR. During extraction, that optional path rendered blank in WebGPU without console errors, so it needs a separate focused follow-up instead of broadening this low-risk port.

Why the WebGL path is low risk

• MultiIconLayer returns the same GLSL shader object on WebGL; the WGSL source is attached only on WebGPU.
• bufferGroup metadata is consumed only by the WebGPU AttributeBufferGroups path.
• The example keeps CollisionFilterExtension unchanged on WebGL.

Validation

• yarn build
• yarn lint (passes with the repository's existing 70 module warnings and 18 example warnings)
• yarn test-headless test/modules/layers/text-layer/text-layer.spec.ts
• yarn test-website
• Manually loaded /examples/text-layer with WebGPU selected: glyphs render and the runtime console has no errors.

[coveralls]

coverage: 83.355% (-0.02%) from 83.372% — codex/webgpu-text-layer into master