[cairo] FLush surface on cairo_destroy()

When discussing the implications of snapshot cow, one of the questions
that we raised was what happens on cairo_destroy()? The lifetime of the
context implicitly marks the extents of the drawing operations, typically
the expose event (or perhaps one phase of it). Therefore at the end of the
sequence we implicitly wish to flush the graphics events to the surface.

src/cairo.c

@@ -212,6 +212,8 @@ cairo_reference (cairo_t *cr)
 void
 cairo_destroy (cairo_t *cr)
 {
+    cairo_surface_t *surface;
+
     if (cr == NULL || CAIRO_REFERENCE_COUNT_IS_INVALID (&cr->ref_count))
 	return;
 
@@ -225,6 +227,15 @@ cairo_destroy (cairo_t *cr)
 	    break;
     }
 
+    /* The context is expected (>99% of all use cases) to be held for the
+     * duration of a single expose event/sequence of graphic operations.
+     * Therefore, on destroy we explicitly flush the Cairo pipeline of any
+     * pending operations.
+     */
+    surface = _cairo_gstate_get_original_target (cr->gstate);
+    if (surface != NULL)
+	cairo_surface_flush (surface);
+
     _cairo_gstate_fini (cr->gstate);
     while (cr->gstate_freelist != NULL) {
 	cairo_gstate_t *gstate = cr->gstate_freelist;