Checkerboarding Prevention project. Find places where we checkerboard with APZ, find out why, fix those problems across platforms.

Just making this actually explicit: The goal for this project is to Paint Fast. IE, Safari, and Chrome, even when they invalidate poorly, still paint fast enough that it isn't a huge checkerboarding problem. Therefore, the explicit assumption that we should change is that instead of fixing invalidation bugs, we should just paint fast even in the cases of bad invalidation.

An update on our strategy for this bug: 1) Optimize checkerboarding problems on Windows 7, with an Intel HD graphics card and a d2d 1.1 backend. This is because 60% of our users are on Windows 7 and ~30% are on some form of an Intel HD card. See [A]. This means we will actively prioritize Windows performance over any other platform and will probably not optimize the other platforms much. 2) Once (1) is satisfactory, switch the other backends to skia in bug 1186552. This should mostly already give us some performance boosts across platforms. 3) Once Skia is everywhere except Windows, we'll repeat (1) for the other platforms. [A] http://people.mozilla.org/~danderson/moz-gfx-telemetry/www/

From many of the recent bugs, we've been getting quite nice X performance improvements. From comment 2, we'll move onto step (2), of consolidating all the backends into skia when during step (1), many of the performance improvements become % improvements instead of X improvements.

I have been capturing two lists of checkerboarding issues, one for checkerboarding "a lot" and one for "a bit". You can find both lists here: <https://etherpad.mozilla.org/ehsan-checkerboarding> I am adding new entries to the lists as I find more issues.

Nice, thanks!