(those originating from a thread other than the gpu thread)
glxMakeCurrent is supposed to error out when the context is
still current in another thread. NVIDIA handles that, but mesa3d errors.
git-svn-id: https://dolphin-emu.googlecode.com/svn/trunk@6054 8ced0084-cf51-0410-be5f-012b33b47a6e