Bug 905468 (Part 1) - Make imgStatusTracker state diffing a cleaner process. r=jdm, a=bajaj
Bug 905468 (Part 2) - Replace SyncNotifyDecodeState with DecodeStateAsDifference. r=jdm, a=bajaj
Bug 867183 - Don't recursively notify in FinishedSomeDecoding. r=jlebar, a=bajaj