[Feature Request]: Replace System.Drawing usage

[AristurtleDev]

Purpose

Replace System.Drawing references with a native implementation of the structs needed.

Motivation

System.Drawing is not fully cross-platform; some of it relies on GDI+ which is windows only. Even though AsepriteDotNet only using System.Drawing.Color, System.Drawing.Rectangle, and System.Drawing.Point, there's no need to enforce usage of these. I think it would be a clearer statement that this is cross platform by being truly cross platform, including which parts of .NET are used.

[AristurtleDev]

This is a breaking change. Currently several methods return System.Drawing.Color or System.Drawing.Color[] values. Any existing applications that are using AsepriteDotNet will need to update their code to accept the new native types.