mapi: Make _glapi_get_stub work with "gl" or "mgl" prefix.

mapi: Make _glapi_get_stub work with "gl" or "mgl" prefix.

When USE_MGL_NAMESPACE is defined, _glapi_get_stub will check for the "m"
prefix before trying to skip it, so that "glFoo" and "mglFoo" are
equivalent.

This should let it work with all the places where something calls
_glapi_get_proc_offset with a hard-coded name that starts with the normal
"gl" prefix.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=55552
Signed-off-by: Kyle Brenneman <kbrenneman@nvidia.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Cc: "10.6 11.0" <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 798f260a2f553e339d7f5fc5120bb627893dc740)