make api use a Uint8List instead of imageLib.Image

[Fnalsk]

Uint8List is more universal as a type in dart. If I'm using an image elsewhere that I'd like to use with this package, I'm most likely using it as a File or Uint8List, not an imageLib.Image. As this package is currently written I would have to convert the image to apply a filter, then convert back if I want to use the result somewhere, which isn't clean.