Bug 1003893 - Ensure that the delete of RasterImage occurs on the main thread. r=seth, a=1.4+

Dispatch nsRefPtr<RasterImage> to the main thread by NS_ProxyRelease
to avoid RasterImage from being destructed by the DecodeJob thread.
(If gMultithreadedDecoding is true.)
Therefore, DiscardTracker can safely access RasterImage::Discard()
by the raw pointer in DiscardTracker::Node.