Remove dependency on System.Drawing

[pgodwin]

I'd like to have this run on .NET core so I can run it on my Linux VPS (although it should work under Mono as is).
Remove the dependency on System.Drawing. This is used for the image resizing/resampling and conversion to GIF.

A quick look shows that either ImageSharp (https://github.com/JimBobSquarePants/ImageSharp) or SkiaSharp (https://github.com/mono/SkiaSharp) would be good replacements.