3D copyToTexture WebGPU/WebGLBackend functionality

[zonkypop]

Description

Currently there is no functionality for copying 3D textures,

I noticed the type signature changed from Box3/Box2 to Vector4 for the srcRegion. I have kept Box3/Box2 so the functionality is the same as the original WebGL renderer.

Copying 3D textures is really useful, I'm currently porting over a distance field ambient occlusion project to TSL and this functionality is needed.

Solution

I've created two PRs for this:

#30618
#30584