Image Improvements

Fixed some image and turtle bugs in #360 and #351. Added Pillow support in #444 and highlighted image diffs in red or yellow in #520. Compare smeared images in #498 and report max diff in #533. Also fixed PyCharm bugs in #528 and #536. Fixed some bugs found by fuzzing in #538 and #539.

Includes PyCascades sprint contributions from @mtkalms, @evelyndai, and @tj-26. Thanks to them and all the people who participated in the sprint.

See the whole list of issues in the milestone.