WebGLBackend: Honor depth/stencil in blitFramebuffer().

[Mugen87]

Related issue: #30995

Description

This PR is one step towards full MSAA support for RTT with the WebGL backend of WebGPURenderer.

The depth and stencil data were never honored during the blit operation so far. That explains why examples like webgpu_custom_fog_background failed with PassNode using MSAA since getViewZNode() or getLinearDepthNode() returned nodes operating on empty buffers.

The WebGL backend uses the same approach like in WebGLRenderer to avoid this issue:

three.js/src/renderers/webgl/WebGLTextures.js

Lines 2137 to 2145 in c0854e6

	if ( renderTarget.resolveDepthBuffer ) {
	if ( renderTarget.depthBuffer ) mask |= _gl.DEPTH_BUFFER_BIT;
	// resolving stencil is slow with a D3D backend. disable it for all transmission render targets (see #27799)
	if ( renderTarget.stencilBuffer && renderTarget.resolveStencilBuffer ) mask |= _gl.STENCIL_BUFFER_BIT;
	}

/cc @cabanier

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	337.4 78.67	337.4 78.67	+0 B +0 B
WebGPU	553.83 153.48	553.93 153.5	+103 B +15 B
WebGPU Nodes	553.18 153.33	553.28 153.34	+103 B +15 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	468.59 113.33	468.59 113.33	+0 B +0 B
WebGPU	628.8 170.19	628.9 170.21	+103 B +18 B
WebGPU Nodes	583.65 159.53	583.76 159.55	+103 B +15 B

[cabanier]

LGTM! Will you also add the option to use multisampled renderbuffers for platforms that don't support render-to-texture?

[Mugen87]

What platforms don't support this workflow?

[cabanier]

What platforms don't support this workflow?

I believe only Quest and Pico browsers support this so Chrome and Safari need MSAA renderbuffers. We need to render to a MSAA texture because that is the only way to get foveation to work.

[Mugen87]

Understood. At the moment the goal is to fix #30995 so MSAA with post processing and MRT works with the WebGL backend. I'll work on the MSAA/MRT combination next.