Slow and/or bad conversion of RAW images to JPEG

[hjbaader]

I plan to save only RAW images rather than the camera generated JPEG in the future. Making such images into gallery images might work with some formats, but it is slow because a giant TIFF file or whatever ist generated by Pillow. For my images in CR2 format, it produces mostly trash.

However the CR2 files contain a full-sized JPEG preview of the image in acceptable quality. So we could create a gallery image by simply extracting this JPEG and resizing it. I have created a patch which works for me but needs to be improved because it hard-codes everything. I'll make a pull request for checking it out.

[saimn]

For the record, from the discussion in #414:

I would prefer to avoid dependency on external tools though, and I think there is a pretty good Python alternative: https://github.com/letmaik/rawpy
rawpy is based on libraw so it supports a wide range of raw formats (CRW/CR2, NEF, RAF, DNG, and others), and the README has an example of how to extract the preview image.