refactor(pandagl): unify rounded border and crop corner rendering

[Copilot]

Pull request created by AI Agent

[codecov]

Codecov Report

❌ Patch coverage is 40.27778% with 86 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.40%. Comparing base (dac5283) to head (13d13ed).

Files with missing lines	Patch %	Lines
lib/pandagl/src/border.c	40.27%	84 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #327      +/-   ##
===========================================
+ Coverage    61.60%   62.40%   +0.79%     
===========================================
  Files          194      194              
  Lines        18454    18181     -273     
  Branches      2478     2413      -65     
===========================================
- Hits         11369    11345      -24     
+ Misses        6204     5960     -244     
+ Partials       881      876       -5     

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