GRAPHICS: Make JPEGDecoder request RGB output from libjpeg by default.

This fixes loading of JPEG files which contain RGB color space instead of YUV.
It is a pretty odd extension of JPEG files by Adobe which is indicated by this:
http://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/JPEG.html#Adobe

To still support Groovie's need for YUV data I added some possibility to
request direct YUV output.
GRAPHICS: Add some paranoia asserts in JPEGDecoder.