[Impeller] cleaned up StrokePathGeometry and removed runtime polymorphism

[gaaclarke]

followup to #50379

improvements:

1. Symbols in cc are now in anonymous namespaces
2. The header is smaller (limits recompilation time)
3. Removed runtime polymorphism (faster, yay)

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[gaaclarke]

You could probably improve this even further by removing the functions pointers now that things are templated.

[jonahwilliams]

LGTM, thank you!

Should be test exempt as its a refactor/non-semantic change that is already tested

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