gl_marshal.py: rework how the marshal dispatch table is initialized

gl_marshal.py: rework how the marshal dispatch table is initialized

Instead of setting all function pointers in marshal_generated0.c,
set the function pointers in the file that contains the functions,
and remove all the forward declarations of marshal functions
in marshal_generated.h.

Acked-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18199>