WebGPURenderer: Fix clear alpha in WebGLBackend

[RenaudRohlinger]

Description

This PR addresses an issue where the renderer's clear method does not properly clear the alpha channel when autoClear is set to false. Additionally, the viewport and scissor logic in the WebGLBackend has been refactored to align with best practices, reducing unnecessary WebGL bindings.

This contribution is funded by Utsubo

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	339.53 79.09	339.53 79.09	+0 B +0 B
WebGPU	491.02 136.44	491.65 136.62	+633 B +170 B
WebGPU Nodes	490.49 136.34	491.12 136.5	+633 B +168 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	465.39 112.16	465.39 112.16	+0 B +0 B
WebGPU	561.56 152.15	562.2 152.31	+637 B +168 B
WebGPU Nodes	516.95 141.72	517.58 141.88	+637 B +167 B

[Mugen87]

@RenaudRohlinger There seems to be a minor issue with the clearColorValue. Everything works great if you use the default setup but if you set renderer.outputColorSpace to THREE.LinearSRGBColorSpace, a runtime error occurs:

three.webgpu.js:49759 Uncaught TypeError: Cannot set properties of null (setting 'clearColorValue')
at WebGPURenderer.clear (three.webgpu.js:49759:33)
at _display/:67:10

Live example: https://jsfiddle.net/obvwkyne/1/

The linear-srgb workflow skips the internal render target so the clear targets the canvas (default) framebuffer. Consequently, there is no renderTarget set which means renderContext ends up null.

It does work with r172 though: https://jsfiddle.net/obvwkyne/2/

[RenaudRohlinger]

Should we just wrap this line with the !== null then @Mugen87?

if ( renderContext !== null ) {
  renderContext.clearColorValue = this._clearColor;
}

I would guess this PR was a fix for renderTarget and the issue it covers didn't concerned no renderTarget.

[Mugen87]

That sounds good!