Handling cube textures in GLES backend

[kvark]

One of the restrictions listed in #1574 is that we have to choose the cube map target at texture creation, not at view creation.
Using any kind of heuristic for it is quite unfortunate...

I believe there is a robust way to implement it, but it comes at a cost. If we detect at texture creation that it could be a cube, we can create another texture for the cube, internally. Then whenever we see a subresource transition onto SAMPLED, we can perform a copy from the 2D array texture into the Cube one. The cost here is 2x VRAM, plus copy operations.