Make framebuffer bindable

[bit-hack]

A framebuffer should be bindable rather then fixed at nano3d init time.
This would allow render to texture type effects.

The main problem I see is that the framebuffer owns the bins that are processed by the workers. So there is a lot of interference.

Perhaps we need a sync command that waits for all bins to finish rasterizing, and then we can switch framebuffers. This will make framebuffer switching expensive however.