Rasterizing artifacts when drawing large and complex polygons #106

Prerequisites

• I have written a descriptive issue title
• I have verified that I am running the latest version of ImageSharp.Drawing
• I have verified if the problem exist in both DEBUG and RELEASE mode
• I have searched open and closed issues to ensure it has not already been reported

Description

Although rendering is less prone to floating point accuracy issues since #96, it's still possible to stress it to produce unwanted artifacts. It might be a logical corner case unhandled by PolygonScanner instead of a numeric issue.

Steps to Reproduce

Enable and run DrawingRobustnessTests.LargeGeoJson_Lines.

The output has artifacts:
https://user-images.githubusercontent.com/6835152/97231127-5538e200-17db-11eb-9aec-e305a41b0e37.png

System Configuration

• ImageSharp.Drawing version: current master
• Other ImageSharp packages and versions: -
• Environment (Operating system, version and so on): all
• .NET Framework version: all
• Additional information: -