XQuartz: GLX: Create a new dispatch table rather than modifying the existing one

XQuartz: GLX: Create a new dispatch table rather than modifying the existing one

Fixes regression introduced by b0c665ac0fe6840dda581e4d0d0b76c703d62a7b

0   X11.bin                       	0x0000000100118293 __glXAquaScreenCreateContext + 684
1   X11.bin                       	0x00000001001315b0 DoCreateContext + 163
2   X11.bin                       	0x000000010013509f __glXDispatch + 211
3   X11.bin                       	0x00000001000c7dad Dispatch + 785
4   X11.bin                       	0x00000001000b97e5 dix_main + 1022
5   X11.bin                       	0x00000001000122bc server_thread + 50
6   libSystem.B.dylib             	0x00007fff836554f6 _pthread_start + 331
7   libSystem.B.dylib             	0x00007fff836553a9 thread_start + 13

http://lists.apple.com/archives/X11-users/2011/May/msg00045.html

Signed-off-by: Jeremy Huddleston <jeremyhu@apple.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>