Virtualizing paint functions

[Anders Carlsson <andersca imendio com>]

Hello,

for the Quartz backend I'd like to use the native versions of some 
things GTK+ normally provides. These are:

* Double buffering (gdk_window_begin/end_paint)

* Invalidating regions (gdk_window_invalidate_region, 
gdk_window_process_updates)

In order to do that I'd like to make it possible to replace some of the 
functions. I currently have a patch that adds a function call:

void _gdk_set_paint_functions (GdkBeginPaintRegionFunc       
begin_paint_region,
                  GdkEndPaintFunc               end_paint,
                  GdkInvalidateMaybeRecurseFunc invalidate_maybe_recurse,
                  GdkProcessUpdatesFunc         process_updates,
                  GdkProcessAllUpdatesFunc      process_all_updates);

which lets backends (this is for obvious reasons NOT a public API) 
provide their own implementations of some functions.

I'd like to hear opinions on this from other backend maintainers.

Regards,
Anders