Update Graphics/Gloss/Juicy.hs

This patch is not tested (not even compiled...), but it might do the trick, and it should be _really_ faster as we don't have to repack a BMP (and Juicy.Pixels is really slow at BMP encoding right now :])
Merge pull request #1 from Twinside/patch-1

Update Graphics/Gloss/Juicy.hs