Speed up scan-line rendering using "Active Edge Table" #90

Currently, brute force search is being used when calculating intersections, the scanline is being tested against all lines in the polygon for each row. This means O(h*n) intersection tests where h is the number of horizontal edges and n is the number of lines.

The fastest way to achieve significant improvement with a (relatively) scoped change is to refactor the intersection detection logic to use an "Active Edge Table". In short this means:

• Sorting edges by both minimum y and maximum y
• Maintaining a list of edges which are currently intersected ("active") while iterating the scanlines
• .. so intersection tests are no longer needed, it's enough to calculate intersection points only for edges which are active.

This should reduce the number of intersection calculations to O(n*log(n) + h*p), where p is the highest number of intersection points in a row during the scan.

Some sources describing the algorithm:

• https://www.techfak.uni-bielefeld.de/ags/wbski/lehre/digiSA/WS0607/3DVRCG/Vorlesung/13.RT3DCGVR-vertex-2-fragment.pdf
• https://www.geeksforgeeks.org/scan-line-polygon-filling-using-opengl-c/#:~:text=Scanline%20filling%20is%20basically%20filling,the%20vertices%20of%20the%20figure.

Not in scope

Improving accuracy further (aka #5 and #15) is not my main goal here, the only criteria is to not regress. But I hope I can achieve better results, or at least get some insights.

Plan

1. Expose an API on IPath returning tessellation results to be consumed by a horizontal scanline rasterizer. These should be stored in a memory-efficient, cache friendly form. A data structure equivalent to (PointF, Orientation)[][] could probably do the job, where Orientation is always assuming a horizontal scanline crossing the point.
2. Introduce a separate ScanlineIterator class that takes the tessellation, and exposes an API to iterate the horizontal scanlines.
3. Utilize ScanlineIterator in drawing code where we currently use FindIntersections, particularly: FillRegionProcessor<T>, DrawTextProcessor<T>