lock-surface: Set geometry width and height before centering

The subsurface patches changed the center_on_output() behavior a bit.
Instead of using the buffer width and height, it now looks at surface
geometry.  In lock_surface_configure() we haven't set up the geometry
when we call center_on_output() so the lock surface would never show
up.

src/shell.c

@@ -2451,6 +2451,8 @@ lock_surface_configure(struct weston_surface *surface, int32_t sx, int32_t sy, i
 	if (width == 0)
 		return;
 
+	surface->geometry.width = width;
+	surface->geometry.height = height;
 	center_on_output(surface, get_default_output(shell->compositor));
 
 	if (!weston_surface_is_mapped(surface)) {