…=nical,Bert

Simplify some of the logic related to handling multiple
compositor surfaces in future, specifically:

* Only rebuild the tiles map when the tile rect changes.
* Remove need for tiles_to_draw array.

Differential Revision: https://phabricator.services.mozilla.com/D62694

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/0c0eb80974b03f6cc153616fe21c8dc229b25c3e