Squashed from the PR, relevant commit messages follow below:
Added file size option to artresizer
- In line with comments on PR, adjusted the ArtResizer API to add
functionality to "resize to X bytes" through `max_filesize` arg
- Adjustment to changelog.rst to include max_filesize change to ArtResizer
and addition of new plugin.
Added explicit tests for PIL & Imagemagick Methods
- Checks new resizing functions do reduce the filesize of images
Expose max_filesize logic to fetchart plugin
- Add syspath escaping for OS cross compatibility
- Return smaller PIL image even if max filesize not reached.
- Test resize logic against known smaller filesize (//2)
- Pass integer (not float) quality argument to PIL
- Remove Pillow from dependencies
- Implement "max_filesize" fetchart option, including
logic to resize and rescale if maxwidth is also set.
Added tests & documentation for fetchart additions.
Tests now check that a target filesize is reached with a
higher initial quality (a difficult check to pass).
With a starting quality of 95% PIL takes 4 iterations to succeed
in lowering the example cover image to 90% its original size.
To cover all bases, the PIL loop has been changed to 5 iterations
in the worst case, and the documentation altered to reflect the
50% loss in quality this implies. This seems reasonable as users
concerned about performance would most likely be persuaded to
install ImageMagick, or remove the maximum filesize constraint.
The previous 30% figure was arbitrary.
