libGLdispatch: Race condition in PatchEntrypoints

Created by: kbrenneman

libGLdispatch will try to let a vendor library patch the OpenGL entrypoints when there's a single thread with a current context. When a second thread calls MakeCurrent, then it'll restore those entrypoints to the original versions.

That means that the second thread might try to rewrite an entrypoint while the first thread is executing it.