…taro,nical

There are a number of issues with the current gradient dithering
implementation, that cause many test failures and also fuzziness
rendering when enabling DirectComposition virtual surfaces. In
particular, the dither result is dependent on the offset of the
update rect within a render target.

For now, this patch disables gradient dithering by default. This
gives us:

 - A heap of new test PASS results (or reduced fuzziness).
 - Fixes a number of non-deterministic fuzziness bugs with DC.
 - Improves performance of gradient rendering by a reasonable amount.

We can fix gradient dithering as a follow up, and re-enable if/when
we find content that would benefit from it significantly (we may
be able to improve gradients in other ways than dithering too).

Differential Revision: https://phabricator.services.mozilla.com/D60460

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/f758f235dd70f1060dba771ccfd3cb91a9aaef20