createOffscreenContext left the new context bound to the calling thread
causing a race condition with the background thread doing the actual
rendering. see T55555 for a more detailed description of the problem.
this patch changes the behavior of createOffscreenContext to restore the
context to the calling context.
Reviewers: fclem
Differential Revision: https://developer.blender.org/D3499
Ray molenkamp
2fa231a86b