Unagi Compositing Manager

Optimize window repainting area (closes: #26).

Before, DamageReportNonEmpty was used but this was not efficient at all as it
just reports a single DamageNotify event each time the damage rectangle
changes from empty to non-empty, meaning that the damaged area will be the
full window.

Instead, use DamageReportRawRectangles to get the real damaged area of a
window. This also makes check whether a window is fully damaged more useful.

Use DamageReportDeltaRectangles to avoid being flooded with DamageNotify (refs: #26).

With DamageReportRawRectangles level, no attempt to compress out overlapping
rectangles is made, therefore many events are received and handled needlessly
(according to a test, 3 times more events are received on the same test).
Whereas with DamageReportNonEmpty level only a single event specifying the
full window region is sent thus this is not efficient for small damage
regions.