Back out 171c124a1402 (bug 961712) for debug b2g mochitest crashes after timeouts
Bug 961783. Remove unneeded DDB optimization. r=vlad

As far as I can tell we always draw images to dc's that are in system memory.
In this case there's no upside to having ddb and a potential downside because
we have to do a readback from gpu into system memory.

--HG--
extra : rebase_source : e536721adc1de2bcff3300ee29c2d1c4b8c15ac0