st: Fix painting unblurred texture

ClutterActorBox encapsulates (x1, y1, x2, y2), but the
second pair of values to the texture node are width and
height, leading to incorrectly rendering the unblurred
texture.

Fix that by properly making these values be x2 and y2.

Closes https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/3497

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1542>

src/st/st-private.c

@@ -443,7 +443,8 @@ _st_create_shadow_pipeline (StShadow    *shadow_spec,
   clutter_paint_node_add_rectangle (texture_node,
                                     &(ClutterActorBox) {
                                       half, half,
-                                      src_width, src_height,
+                                      src_width + half,
+                                      src_height + half,
                                     });
 
   paint_context =