This defines several callbacks to be used by vendor libraries for
rewriting top-level entrypoints defined in libglvnd:

- initiatePatch(): libglvnd calls into this function at make current
  time to determine whether entrypoint patching is possible with the
  given stub type and code size.  The vendor library will notify
  libglvnd whether offsets are needed based on a generation number.
- getOffsetHook(): libglvnd calls into the vendor library with this hook
  for each loaded interface library so the vendor can lookup the stub
  locations for rewriting.
- finalizePatch(): libglvnd calls into this function after the vendor
  library has updated its stub locations to complete the patch.