Fix framebuffer completeness being broken after last framebuffer cleanup
commits.

Basically, we don't set a draw buffer until draw time comes. Also add
explicit validation function to validate after all textures have been
attached (could be done automatically at bind time too probably, but
left out for now)
Make sure world redraws correctly when we tweak the world properties
Minor UI message fix.
BLI_bitmap: add allocation from a MemArena.