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

[Copilot]

Pull request created by AI Agent

[codecov]

Codecov Report

❌ Patch coverage is 42.58555% with 151 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.68%. Comparing base (dac5283) to head (55a96ed).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
lib/pandagl/src/border.c	42.58%	144 Missing and 7 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #327      +/-   ##
===========================================
+ Coverage    61.60%   62.68%   +1.07%     
===========================================
  Files          194      194              
  Lines        18454    18331     -123     
  Branches      2478     2454      -24     
===========================================
+ Hits         11369    11490     +121     
+ Misses        6204     5962     -242     
+ Partials       881      879       -2     

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