Would be nice to have on-device copy from 2D to 3D texture.
You can bind the z-slices of a 3d texture as FBO, and render with 2D texturing to them.
No, that is not what I want.
I want to copy 2D texture into the slice of a 3D texture.
Just copy, no rendering!
You can blitt it and copying with a PBO should work too.