Prevent PathGradientBrush from throwing an error with corner cases

[mysticfall]

Prerequisites

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• I have provided test coverage for my change (where applicable)

Description

While testing the initial implementation of PathGradientBrush with a more complex scenario, I found that there are such cases where it fails to find intersection points for a given position.

The previous code assumed it would be always possible to calculate an intersecting point of a line between the center position and an arbitrary point within the paint area. But it seems that the assumption was wrong for very small polygons, for a reason that I have yet to understand perfectly.

So, I just made a small correction to ignore such corner cases which prevents an error from halting the paint process. And it looks like ignoring them doesn't affect the output image noticeably.

As such, I think we could address this problem with the proposed change without breaking any existing code (that is, if anyone beside me is already using the new gradient brush), although further investigation might be needed in future.

[mysticfall]

Not sure why one of the build fails, but I think it's not related to the changes I made?

[codecov]

Codecov Report

Merging #1007 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1007   +/-   ##
=======================================
  Coverage   89.85%   89.85%           
=======================================
  Files        1098     1098           
  Lines       48863    48863           
  Branches     3433     3433           
=======================================
  Hits        43908    43908           
  Misses       4254     4254           
  Partials      701      701

Impacted Files	Coverage Δ
...ImageSharp.Drawing/Processing/PathGradientBrush.cs	90.9% <100%> (ø)	⬆️

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[codecov]

Codecov Report

Merging #1007 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1007   +/-   ##
=======================================
  Coverage   89.85%   89.85%           
=======================================
  Files        1098     1098           
  Lines       48863    48863           
  Branches     3433     3433           
=======================================
  Hits        43908    43908           
  Misses       4254     4254           
  Partials      701      701

Impacted Files	Coverage Δ
...ImageSharp.Drawing/Processing/PathGradientBrush.cs	90.9% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5b73284...1eea497. Read the comment docs.

[mysticfall]

Also, I changed type of the constructor argument from ILineSegment[] to PointF[].

The reason is, users would normally start with points and their associated colors, so it can be quite cumbersome to construct line segments by hand. The equivalent class in .NET API (System.Drawing.Drawing2D) also expects an array of points, so I think this change could make the API look more intuitive.

Also, I realized that LinearLineSegment can potentially have more than 2 points, and the case was not considered in the original codebase.

I'm not sure if I can make a breaking change to the existing API at this stage. So, if it's not allowed, I'll update the PR without the relevant part.

[tocsoft]

Thanks for the PR 👍.
I would agree and think the breaking change is fine in this case.

[mysticfall]

Thanks for the quick review! :)