GTK: fix small arc rendering problem

[knocte]

GeometryHelper.FlattenArc function returns a NaN point when
flattening very small arcs. Cairo doesn't ignore NaN points
and would draw an out-of-place line instead, which we need
to tweak to prevent weird renderings.

Co-authored-by: Afshin Arani [email protected]

Description of Change

Issues Resolved

• fixes [Bug] (Gtk) Cheese chart ends up having weird rendering  #15130

API Changes

None

Platforms Affected

• Gtk

Behavioral/Visual Changes

None

Before/After Screenshots

Not applicable

Testing Procedure

PR Checklist

• Targets the correct branch
• Tests are passing (or failures are unrelated)

[jfversluis]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[jfversluis]

Hey @knocte thanks for another great contribution! Would you be able to test the resulting NuGet from this build to verify if it has the desired outcome? Thanks!

[knocte]

@jfversluis do you ask me that in case we haven't tested the fix? We did test the fix, even with our own nuget package (https://www.nuget.org/packages/DotNetForms.Platform.GTK), because we cannot wait for Microsoft to release next SR, so we create it ourselves :)

[jfversluis]

I assume that you're testing it! I would love for you to test it out with the NuGet that is on this PR to verify that it also works in this case and nothing got mixed up somehow in between. The instructions are here. The version for this case is 5.0.0.8026. If you would be willing to double-check that would be greatly appreciated!