The main goal is to allow more flexibility, allow cubemap arrays in
the future and better match vk and metal apis.
Main changes:
- remove updateCubeImage
- remove update2DImage
- update3DImage is now the only texture upload backend API
cubemaps are now treated just like a 2D array of 6 layers.
For this reason, Texture::setImage(..., FaceOFfsets) is deprecated.
Additionally, the 2D versions of Texture::setImage() become inline
helpers.
A side effect of this change is that it is now possible to update only
a single face of a cubemap, but also a region of a face (or faces).
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