Fix casting of list of textruns

[stefannikolei]

Directly casting is not possible. That's why using the Cast method is the easiest way

Fixes #332

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 80%. Comparing base (ac272a2) to head (ce0f50c).
Report is 13 commits behind head on main.

Files	Patch %	Lines
...c/ImageSharp.Drawing/Processing/RichTextOptions.cs	0%	1 Missing ���️

Additional details and impacted files

@@         Coverage Diff         @@
##           main   #333   +/-   ##
===================================
  Coverage    80%    80%           
===================================
  Files        97     97           
  Lines      4960   4971   +11     
  Branches    886    888    +2     
===================================
+ Hits       4011   4023   +12     
+ Misses      754    753    -1     
  Partials    195    195           

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unittests	80% <0%> (+<1%)	⬆️

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[JimBobSquarePants]

Would direct casting not work if we explicitly set an empty array as the default value?

[antonfirsov]

This creates a copy and in my understanding Cast should also fail since the default instance is has the property assigned to TextRun[] not RichTextRun[].

Would direct casting not work if we explicitly set an empty array as the default value?

That would be a better solution IMO, but it would allow bringing the object into an invalid state by downcasting RichTextOptions to TextOptions. It is an unusual but valid code.

RichTextOptions a = new(font);

Foo(a);
void Foo(TextOptions b) =>  b.TextRuns = new TextRun[0];

IReadOnlyList<RichTextRun> runs = a.TextRuns; // exception

[JimBobSquarePants]

I think we'd probably fix that with some sort of generic pattern in the future but for now our options are limited.