More flexibility with gpu.types.GPUOffScreen

The gpu.types.GPUOffScreen class is very handy especially because reproducing an equivalent using raw bgl functions such as glGenFramebuffer and co leads to random crashes of Blender. But it is quite limited, in particular it is not possible to setup several color attachments, nor to chose the bit depth.

Enabling the use of 16-bit attachements would be possible by adding a python argument instead of the hard-coded false here in bpygpu_offscreen_new I guess.

But for multiple color attachments I am less sure. Plus, before starting this, I’d like to know if there isn’t already a way to do so that I am not aware of, and whether this would be something likely to be merged upstream.