compositor: Set pointer surface input region to empty on attach

We set up an emtpy region for the initial NULL-cursor, and then going
from no buffer to a 32x32 buffer didn't trigger the undeffing of the input
region.  So when something sets a cursor of a different size, the cursor
surface input region gets initialized and starts swallowing events.

src/compositor.c

@@ -1959,6 +1959,7 @@ input_device_attach(struct wl_client *client,
 		buffer = buffer_resource->data;
 
 	weston_surface_attach(&device->sprite->surface, buffer);
+	empty_region(&device->sprite->input);
 
 	if (!buffer)
 		return;