Bug: DrawLine doesn't draw pixels where line segments overlap.

[JimBobSquarePants]

Discussed in #339

^{Originally posted by woutware September 3, 2024}
Tested with SixLabors.ImageSharp 3.1.5, SixLabors.ImageSharp.Drawing 2.1.4, .NET 6.0:

Program to reproduce (Only draws line from (10, 10) to (20, 10). The part where it draws over itself is not drawn):

using System;

using SixLabors.ImageSharp.PixelFormats;
using SixLabors.ImageSharp;
using SixLabors.ImageSharp.Processing;
using SixLabors.ImageSharp.Drawing.Processing;
using SixLabors.Fonts;

namespace ConsoleApp {
    public static class SixLaborsBug {
        public static void DrawLineBug2() {
            Image<Bgra32> bitmap = new Image<Bgra32>(Configuration.Default, 100, 100, new Bgra32(0, 0, 0));
            // Only draws line from (10, 10) to (20, 10). The part where it draws over itself is not drawn.
            bitmap.Mutate(
                context => {
                    context.DrawLine(Color.Aqua, 1f, new PointF(10, 10), new PointF(90, 10), new PointF(20, 10));
                }
            );

            bitmap.Save(@"C:\tmp\DrawLineBug2.png");
        }
    }
}

Attached is the output bitmap.