Add a postprocessing utility to replace dcraw/convert/exiftool
Heavily based on postprocessd: https://git.sr.ht/~martijnbraam/postprocessd
Also, output half the resolution when processing.
This dramatically reduces processing time. The resulting photos and videos are 50% the original resolution, but since we were already using linear interpolation to make processing faster the loss of quality shouldn't be very noticeable. In case of photos, there's also the original DNG retained which can be used to high quality processing afterwards.
Fixes #64 (closed)