Attempt to support Proraw dng

[ilia3101]

This is a very minimal draft, I intend to make any necessary improvements.

Changes:

• Duplicated decode_ljpeg_3components and made the smallest change possible to work with proraw compression.
• Allow down to 10 bit precision, as some ProRaw files are 10-bit.

I know this could be improved a lot.

Please give feedback/suggestions.

I'm also interested in implementing the other ljpeg predictors if I can find any samples.