System.Drawing assembly is not supported on non-Windows platforms

[RangHo]

Starting from .NET 6, System.Drawing assembly that this script depends on no longer supports platforms other than Windows. In .NET 7, the compile-time switch that allows System.Drawing.Common package to be used is removed. See this link for more info.

Since this raytracer depends on System.Drawing for PNG output, we need to address this issue. There are two possibilities:

1. Introduce third-party image processing libraries, such as SkiaSharp;
2. Revert back to PPM output just like the OG Raytracing Book does.