It has been a while since the last optipng run (it was in 2015),
and many non-optimized PNG files have accumulated in the repository.
288 of 1666 files optimized, 258992 bytes reduced to 156258 bytes; -102734 bytes, -39.66%
To avoid unnecessarily increasing the size of Git repository for minor
size reductions, we skip images that weren't reduced by at least 15%.
By taking images/misc/orb@2x.png and editing it (in non-trivial ways,
sadly), we get images that are more suitable for the UI than what we had
up to this point, without the result looking upscaled and ugly, and
without requiring a hardcoded IPF full of magic numbers.
I drew the frame for the pressed variation myself using the pressed
slider block thingy from images/buttons/sliders/slider-pressed.png.
