MozJPEG lossless transform

I don't know if this should be V1 or not…

Some JPEG encoders support lossless transforms, such as MozJPEG. We should enable this.

Currently our encoders take image data, so we need to figure out when it's acceptable to pass through the source file directly.

The options UI may need to be aware of when lossless transforms are possible.

Start by tracing how encoders receive image data and how the options UI determines available operations. Investigate where the source file could be passed through for MozJPEG lossless transforms, then define and verify the conditions under which that path and its UI options are valid.

Written by the indexing model from the issue text.