The issue was caused by a bad usage of GPUOffscreen.
The Framebuffer was created using a window framebuffer and used
in a viewport callback when another GPUContext was bound.
This change allows up to 3 framebuffers per GPUOffscreen.
Most common case will be using 2 framebuffers (one for init and
one for drawing) but in the case of more (bad usage) it will just
degrade performance a bit.
5e626e7664