glx/apple: locate dispatch table functions to wrap by name

Avoid reaching into the dispatch table internals (and thus having to deal
with the complexities of remap etc.) by identifying functions to wrap by
name.

See:
https://lists.freedesktop.org/archives/mesa-dev/2015-June/086721.html et seq.
https://bugs.freedesktop.org/show_bug.cgi?id=90311



Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
(cherry picked from commit d3540b40)