Bug 1581748 - [Wayland] Fixed rendering of overlapping areas, r=jhorak

- Don't store whole buffer update information at mWholeWindowBufferDamage but use damage region instead.
- Check and remove overlapped images when they are stored at image cache.
- Remove CanDrawToWaylandBufferDirectly() as it's not very useful.

This commit depends on Bug 1580152 which needs to land first.

Differential Revision: https://phabricator.services.mozilla.com/D46140

--HG--
extra : moz-landing-system : lando