Reworked the __GLdispatchPatchCallbacks interface, mostly toward fixing issue
37.

__GLdispatchPatchCallbacks::initiatePatch now handles the entire patching
process, instead of splitting it between initiatePatch, getOffsetHook, and
finalizePatch.

Added callbacks to __GLdispatchStubPatchCallbacks that are called before and
after calling into the vendor library to patch the entrypoints.

Changed the documentation to say that the vendor library can only change the
entrypoints during the call to initiatePatch. That'll allow libGLdispatch to
change the memory protection before and after the app is finished modifying
the entrypoints.

The callback that a vendor library calls to fetch an entrypoint now returns
separate writable and executable addresses.