Convert P images with transparency into RGBA

Fixes #800
do not convert images with palette-based transparency to RGBA, as that kills the background colour
Issue with transparency handling also affects image_data_as_rgb, so make the fix into a shared _to_rgb helper method
set a non-black background colour on Jcrop so that transparent images don't look horrible
Version bump Pillow to avoid #800
Changelog/release note for #808