Since desktop/ code queues, compresses and only emits callbacks on idle,
it's possible that two invalidations are in the queue, and there was a
setPart() call between them. In this case it's impossible to tell what
part the invalidation was sent for.

Fix this by conditionally including the part number in the invalidation
payload. It's off by default, a new feature flag is added to request
this behavior.

gtktiledviewer enables this feature flag by default, though just to show
the part number in the debug output. Android doesn't enable it.

Change-Id: I73e6def848c0eb61d64e71026002c7a0e750aab4