gtk-demo: Fix drawing of gestures demo feedback on CSD windows

It was wiping the translation initially applied to the cairo_t, so
pick that up first before applying our own matrix changes.

demos/gtk-demo/gestures.c

@@ -86,9 +86,10 @@ drawing_area_draw (GtkWidget *widget,
       cairo_matrix_t matrix;
       gdouble angle, scale;
 
-      cairo_matrix_init_translate (&matrix,
+      cairo_get_matrix (cr, &matrix);
-                                   allocation.width / 2,
+      cairo_matrix_translate (&matrix,
-                                   allocation.height / 2);
+                              allocation.width / 2,
+                              allocation.height / 2);
 
       cairo_save (cr);