glx: Pull get_proc_address out of the context vtable

glx: Pull get_proc_address out of the context vtable

One, only applegl needs this. Two, this isn't a function of the context
in any way so it doesn't belong in the context vtable. Just special-case
applegl for now, we can revisit applegl's dispatch later.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9690>