Fix IndexOutOfRangeException when drawing lines outside of image

[JimBobSquarePants]

Prerequisites

• [x] I have written a descriptive pull-request title
• [x] I have verified that there are no overlapping pull-requests open
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• [x] I have provided test coverage for my change (where applicable)

Description

Updates PolygonScanner to correctly handle scanlines outside of the image frame. Lines are still centered correctly. Fixes #108

Angle starting and finishing outside of the frame

Stroke 3 Top

Stroke 3 Bottom

Stroke 3 Left

Stroke 3 Right

[codecov[bot]]

Codecov Report

Merging #119 (15dad17) into master (5b168f9) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #119   +/-   ##
=======================================
  Coverage   71.05%   71.05%           
=======================================
  Files          86       86           
  Lines        5472     5472           
  Branches     1119     1119           
=======================================
  Hits         3888     3888           
  Misses       1359     1359           
  Partials      225      225           

Flag	Coverage Δ
unittests	71.05% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...arp.Drawing/Shapes/Rasterization/PolygonScanner.cs	97.75% <100.00%> (ø)

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