pixman-renderer: share region_global_to_output()

Converting a region from global coordinates to output pixel coordinates
will become useful in GL-renderer soon, so move this function to be
shared. It is tricky to reinvent.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>
diff --git a/libweston/backend.h b/libweston/backend.h
index 8e91746..b9afb2c 100644
--- a/libweston/backend.h
+++ b/libweston/backend.h
@@ -170,6 +170,10 @@
 				   double device_x, double device_y,
 				   double *x, double *y);
 
+void
+weston_output_region_from_global(struct weston_output *output,
+				 pixman_region32_t *region);
+
 /* weston_seat */
 
 void