Modify __glDispatchMakeCurrent() to take a pointer to the patch
callbacks for the vendor library whose context is about to be current.

Implement GLdispatch.c:PatchEntrypoints() and helpers.  This contains
the logic for determining whether it is safe to patch entrypoints, and
using the provided vendor callbacks to actually do the entrypoint
patching.  Call this from __glDispatchMakeCurrent() with the provided
patch callbacks.

Also implement GLdispatch.c:CurrentEntrypointsSafeToUse(), which
determines based on the current vendor ID whether the current top-level
code is safe to use with that vendor.  If this check fails, the make
current operation fails.

Allow users to disable entrypoint rewriting functionality by setting the
environment variable __GLVND_DISALLOW_PATCHING=1.

When losing current, also attempt to restore the default libglvnd
entrypoints so other vendors can potentially use them.