Bug 854795. Add support for -moz-samplesize. r=seth

-moz-samplesize allows decoding at a lower resolution.
This is valuable for saving memory when we know that
we don't need a large version of the image.

--HG--
extra : rebase_source : 0373c23bd4a8b5963329ea1ccf13f075c1264909
Automatic merge