TODO

Series-based resizing would be faster if implemented directly
Handle fixed-point overflow in GM fixed
Small memory leak in giflib
BMP RLE support
BMP OS/2 format support
Fix BMP flipped images
Some older versions of giflib don't have DGifGetExtensionNext, require minimum version
Transparent SBS icons get a transparent background with gd_fixed for some reason
Remove GM code, the filters are wrong and it's not faster anyway
Display the filename even if passing in raw data (embedded images)

Tests:
  Corrupt/invalid files in all formats, triggering longjmp's
  PNG <8-bit gray
  JPEG invalid marker, should continue (Michael's image)

Research/benchmark other image libraries:
  http://gmic.eu/ (based on http://cimg.eu/)
  https://www.lrde.epita.fr/wiki/Olena
  http://www.vips.ecs.soton.ac.uk/index.php?title=VIPS
  http://core.tcl.tk/akupries/crimp/wiki?name=References
  Image::Imlib2
  Image::Resize::OpenCV
  PDL

Update benchmarks, include memory benchmarks.