Fix GPUParticles2D ignoring AtlasTexture region

[Geometror]

Fixes #13923.
Also fixes GPUParticles2D's texture not updating in some cases.

[Calinou]

Out of curiosity, do you know if a similar approach can be used for GPUParticles3D?

[Geometror]

@Calinou The problem is that GPUParticles3D and CPUParticles3D are using regular meshes with normal materials, so the logic would have to go in BaseMaterial3D. As BaseMaterial3D supports many different kind of textures (normal, detail, roughness,...) I think this would get quite complicated or it wouldn't be feasable at all since you can't guarantee that every texture is an AtlasTexture.

[kleonc]

It won't work for AtlasTextures with margins. To properly take margins into account points would also need to be changed (and it would need to be taken into account in uvs as well after rethinking uvs would remain the same). Also would be nice if these would be calculated iteratively so nested AtlasTextures would be supported too.

[clayjohn]

This matches the behaviour of CPUParticles2D and is a big improvement over doing nothing.

To properly take margin into account, we will need to make further changes to how UVs are computed by both CPUParticles2D and GPUParticles2D, but that can be done in another PR

[akien-mga]

Needs rebase.

[Geometror]

Rebased.

[akien-mga]

Thanks!

[Mickeon]

My condolences!