Artwork resizing

In imlib:
- JPEGs get scaled as JPEGS, everything else as PNG;
- JPEG quality is 95.

Native libraries:
- JPEG gets scaled to JPEG at quality 95, PNG gets scaled to PNG, everything else will fail;
- PNG transparency is maintained in the scaled image;
- all PNG images are saved as RGB 16 bit, or with alpha channel at 24 bit;
- bilinear interpolation used for both functions.

Recent commits to master are included in this file and it should be completely up to date.