This change computes the new invalid region of the in progress surface
during the partial update, and asserts that the update region is
sufficient to cover it. This will assert earlier than the asserts in
NotifySurfaceReady, under similar conditions. Hopefully this will help us
identify if we are using an incorrectly calculated update area.
Additionally, it reorganizes the region boolean logic in
NativeLayerCA::HandlePartialUpdate to avoid adding in the update region
just to subtract it away again.
It also confirms that callers to the layer are always either using the
drawn surface call (NotifySurfaceReady) or the external surface call
(AttachExternalImage).
Differential Revision: https://phabricator.services.mozilla.com/D169019
79eee79462