Bug 958758. If we need to re-decode an image with different flags and shutting down the existing decoder has caused us to finish decoding we need to discard the image so we can create a new decoder with the new flags. r=seth a=v1.3t+
Bug 958758. Refactor common image discard checks into one place. r=seth a=v1.3t+
Bug 890743 - Display 0-delay, single loop GIFs instantly. r=seth a=v1.3t+

Do not artificially create a frame delay for GIF files
that do not loop and have an inter-frame delay of 0. Move timeout
calculation to FrameBlender and only access raw timeout of
imgFrame from within FrameBlender.
Bug 962670 - Store decoded images in VolatileBuffers, r=seth,jrmuizel a=v1.3t