compositor: do not recompute size on pointer_set_cursor

This removes the calls to weston_surface_buffer_width/height() from
input.c, which are the last external calls to them.

Instead, use the cached values from weston_surface::width,height. These
have already been set by weston_surface_commit(), because that is the
only way a weston_surface can get a buffer.

src/input.c

@@ -1495,8 +1495,8 @@ pointer_set_cursor(struct wl_client *client, struct wl_resource *resource,
 	pointer->hotspot_y = y;
 
 	if (surface->buffer_ref.buffer)
-		pointer_cursor_surface_configure(surface, 0, 0, weston_surface_buffer_width(surface),
-								weston_surface_buffer_height(surface));
+		pointer_cursor_surface_configure(surface, 0, 0, surface->width,
+								surface->height);
 }
 
 static void