Allow sampling multisampled render targets on WebGPU

[SashaRX]

Allows a multisampled render target to be sampled in a user shader on
WebGPU (textureLoad on texture_multisampled_2d /
texture_depth_multisampled_2d), so applications can implement custom
MSAA resolves the hardware box resolve cannot express.

const renderTarget = new RenderTarget({
    colorBuffer,
    samples: 4,
    bindMultisampled: true
});

device.scope.resolve('msColor').setValue(renderTarget.getMultisampledColorBuffer());

var msColor: texture_multisampled_2d<f32>;
let sample = textureLoad(msColor, pixel, sampleIndex);

Fixes #9059

Changes:

• Multisampled color attachments can opt into texture-binding usage and
  be bound through BindGroupFormat.
• MSAA color is stored (not discarded) when bindMultisampled is set,
  so a later pass can textureLoad the samples.
• WGSL reflection understands texture_depth_multisampled_2d and emits
  the correct multisampled type.

API Changes:

• RenderTarget option bindMultisampled (boolean, default false).
  WebGPU, samples > 1. Ignored if transientColor actually applies.

• RenderTarget#bindMultisampled — resolved flag.

• RenderTarget#getMultisampledColorBuffer(index = 0) — bind this
  Texture, not colorBuffer. Owned by the render target.

  The public bindable texture is the MSAA color attachment.
  Auto-allocated MS depth already has TEXTURE_BINDING, and
  texture_depth_multisampled_2d reflects; a public
  getMultisampledDepthBuffer() is a follow-up.

• BindTextureFormat 7th parameter multisampled (default false).
  Forces no sampler; SAMPLETYPE_FLOAT becomes unfilterable-float.

No example added.

Checklist

• I have read the contributing guidelines
• My code follows the project's coding standards
• This PR focuses on a single change

[Copilot]

Pull request overview

Adds WebGPU support for shader access to multisampled render-target color attachments for custom MSAA resolves.

Changes:

• Exposes bindable multisampled color textures from render targets.
• Adds multisampled texture bind-group and WGSL reflection support.
• Preserves MSAA samples and adds unit coverage.

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/platform/graphics/bind-group-format.js	Adds multisampled texture formats.
src/platform/graphics/null/null-render-target.js	Stubs multisampled color textures.
src/platform/graphics/render-pass.js	Stores bindable MSAA samples.
src/platform/graphics/render-target.js	Adds the render-target API.
src/platform/graphics/texture.js	Tracks texture sample counts.
src/platform/graphics/webgpu/webgpu-bind-group-format.js	Emits multisampled layouts.
src/platform/graphics/webgpu/webgpu-render-target.js	Creates bindable MSAA attachments.
src/platform/graphics/webgpu/webgpu-shader-processor-wgsl.js	Reflects multisampled WGSL textures.
src/platform/graphics/webgpu/webgpu-texture.js	Wraps render-target-owned textures.
test/platform/graphics/bind-group-format.test.mjs	Tests format behavior.
test/platform/graphics/render-pass.test.mjs	Tests MSAA storage.
test/platform/graphics/render-target.test.mjs	Tests the public API.
test/platform/graphics/webgpu/webgpu-bind-group-format.test.mjs	Tests WebGPU layouts.
test/platform/graphics/webgpu/webgpu-render-target.test.mjs	Tests texture usage flags.
test/platform/graphics/webgpu/webgpu-shader-processor-wgsl-compute.test.mjs	Tests WGSL reflection.

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[SashaRX]

Agreed — this assert contradicted the documented API.

hasSampler defaults to true, so the natural call new BindTextureFormat(name, visibility, dim, type, undefined, undefined, true) (or just the 7th argument) hit Debug.assert(!hasSampler) in debug builds even though we already coerce this.hasSampler = false.

Removed the hasSampler assert. Coercion stays (multisampled still forces hasSampler = false and samplerName = null). The TEXTUREDIMENSION_2D assert stays — that one is a real WebGPU invariant, not a default-parameter clash.

Shader reflection already strips a following sampler (hasSampler && !resource.multisampled) and asserts at the WGSL source if someone declares sampler after a multisampled texture.

[SashaRX]

Agreed — Texture.samples is not a public way to allocate an MSAA texture, and we should not pretend it is.

WebgpuTexture.create still hardcodes sampleCount: 1 for a normal allocation. The sample count is only meaningful on the wrapper around the render target’s existing MSAA GPUTexture.

What we did:

• Dropped options.samples from the public Texture constructor JSDoc and from API Changes.
• Stopped reading options.samples entirely. new Texture(device, { samples: 4 }) now stays at sample count 1, matching the GPU texture that is actually created.
• The wrapper uses an internal _samples option together with _importedGpuTexture (both @ignore). Texture#samples stays @ignore so getMultisampledColorBuffer() can still describe the imported GPU texture to WebGPU (sampleCount on the adopted descriptor).

We are not adding public multisampled Texture allocation in this PR.