Renderers: Correct the blending formulas

[WestLangley]

Correct the blending formulas for WebGLRenderer, and for both backends of WebGPURenderer.

The NormalBlending formulas are correct, so no changes are needed.

AdditiveBlending had a minor error, which is fixed.

SubtractiveBlending and MultiplyBlending have never been correct. The changes here align with the behavior familiar to Photoshop users.

... more comments later ...

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	337.4 78.67	337.53 78.73	+129 B +52 B
WebGPU	554.99 153.61	555.29 153.68	+302 B +75 B
WebGPU Nodes	554.34 153.46	554.64 153.53	+302 B +74 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	468.59 113.33	468.72 113.38	+129 B +50 B
WebGPU	630.02 170.38	630.32 170.46	+302 B +76 B
WebGPU Nodes	584.87 159.73	585.17 159.81	+302 B +75 B

[WestLangley]

For MultiplyBlending and SubtractiveBlending the opacity of the blend layer controls the intensity of the effect -- not the opacity of the result.

[Mugen87]

I've added a note about this change in the migration guide: https://github.com/mrdoob/three.js/wiki/Migration-Guide#177--178