Make cairo code work with newer cairo backends that actually use caching

Call cairo_surface_flush() before reading or writing a surface's
pixels directly, and use cairo_surface_mark_dirty() after writing
pixels directly, so we don't read old pixels, and our written pixels
get actually used.

libgimpwidgets/gimpcairo-utils.c

@@ -228,6 +228,8 @@ gimp_cairo_surface_create_from_pixbuf (GdkPixbuf *pixbuf)
 
   surface = cairo_image_surface_create (format, width, height);
 
+  cairo_surface_flush (surface);
+
   src         = gdk_pixbuf_get_pixels (pixbuf);
   src_stride  = gdk_pixbuf_get_rowstride (pixbuf);
 
@@ -280,5 +282,7 @@ gimp_cairo_surface_create_from_pixbuf (GdkPixbuf *pixbuf)
       break;
     }
 
+  cairo_surface_mark_dirty (surface);
+
   return surface;
 }