mesa: clean up debug groups

mesa: clean up debug groups

Add struct gl_debug_group to hold all namespaces of a debug group.  Replace
the 3-dimensional array, Namespaces, in struct gl_debug_state by a
1-dimensional array of type struct gl_debug_groups.

Turn the 4-dimensional array, Defaults, in struct gl_debug_state to a
1-dimensional array in struct gl_debug_namespace.

Signed-off-by: Chia-I Wu <olv@lunarg.com>
Reviewed-by: Brian Paul <brianp@vmware.com>