WebGPURenderer: Add per-attachment MRT blending support

[sunag]

Related issue: #32570

Description

The PR adds blending per attachment, which can be useful for custom applications;

The bloom_emissive example has been updated to support transparency using the new approach.

It's likely we'll use bloom_emissive for something that utilizes HDR output; there are still points for visual customization, such as the relationship between the bloom's glow and the object's color, increasing the glow while preserving color nuance. The current example makes things more customizable, which is also a positive thing not only for this but for other things.

const mrtNode = mrt( {
	output: output,
	emissive: vec4( emissive, output.a )
} );

mrtNode.setBlendMode( 'emissive', new THREE.BlendMode( THREE.NormalBlending ) );

scenePass.setMRT( mrtNode );

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	355.34 84.47	355.36 84.47	+17 B +4 B
WebGPU	618.9 171.93	621.44 172.53	+2.54 kB +600 B
WebGPU Nodes	617.5 171.68	620.04 172.29	+2.54 kB +606 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	487.5 119.33	487.5 119.33	+0 B +0 B
WebGPU	691.16 187.57	692.69 187.93	+1.53 kB +355 B
WebGPU Nodes	641 174.81	642.53 175.17	+1.53 kB +361 B