Remember to NULL current colorbuf when setting default

Signed-off-by: Casey Dahlin <cdahlin@redhat.com>
Perform demo lighting via deferred rendering

The hard work is shaders and pipeline setup, but we have to also add a helper
in object.c to get a screenspace quad to draw. I don't suspect it will last
long.

Signed-off-by: Casey Dahlin <cdahlin@redhat.com>
Add LUFT_ prefix to a few constants that were missing it

Signed-off-by: Casey Dahlin <cdahlin@redhat.com>