Instead of having each winsys implement its own list of callbacks the
list is now just attached directly to the CoglOnscreen using code in
cogl-onscreen.c. The winsys's can invoke this list of callbacks by
calling _cogl_onscreen_notify_swap_buffers(). All of the winsys's
would probably have a very similar implementation for this anyway and
I don't think it makes much sense to try and save the cost of a list
pointer in the CoglOnscreen struct.
Reviewed-by: Robert Bragg <robert@linux.intel.com>