Less color conversion

This moves the colour conversion to jpg.js
* The good news: Benchmarking shows that this improves performance for the invitation document
from #3809 by 25%
* There could be problems with just outputting when you find a JPEG. I would appreciate if you could have a look and tell me what you think
* It would probably make sense to move most colorspace conversions into the decoders. This raises questions of the structure of thing ...