In __glDispatchLoseCurrent, don't try to restore the default entrypoints.
Instead, __glDispatchMakeCurrent will check if it's using a different vendor
library, and if so, it will unpatch and repatch the entrypoints then. If it's
using the same vendor library (which will usually be the case), then it can
leave them patched and skip the patch overhead.

Some applications will call glXMakeCurrent to release and bind the same context
every frame, sometimes multiples times. Each patch and unpatch sequnce can chew
up 1-2 milliseconds. That's enough to significantly reduce the framerate of
some applications.

Reviewed-by: James Jones <jajones@nvidia.com>