Added a new function to libGLdispatch, __glDispatchForceUnpatch, which forces
it to unpatch the OpenGL entrypoints before libEGL or libGLX can unload the
vendor library that patched them.

If a vendor patches the OpenGL entrypoints, libGLdispatch doesn't unpatch them
when that vendor's context is no longer current, because that adds too much
overhead to repeated MakeCurrent+LoseCurrent calls. But, that also means that
the patch callbacks end up being dangling pointers after the vendor library is
unloaded.

This mainly shows up at process termination when a process loads both libEGL
and libGLX, because __glxFini and __eglFini will both call the vendor's
threadAttach callback.

Fixes https://github.com/NVIDIA/libglvnd/issues/103