Bug 854803 - Part 1: Account for more decoder state in RasterImage::IsDecodeFinished(). r=seth

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extra : rebase_source : 7fa2c43c5146eca831d5775d1ba4d0c665404626
Bug 854803 - Part 2: Don't try to enqueue more decoding from DecodeDoneWorker. r=seth

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extra : rebase_source : 0d46455743f02d58c5615c72afb90bba13d505bc