refactor to an Expando instead of extending Paint

[blaugold]

Extending Paint makes it impossible to compile for Web.

It's not documented, but extending classes from dart:ui should be
avoided because implementations are often platform-dependent.

[greensopinion]

Thanks for the PR!

Expando is pretty cool.

Did you measure the performance impact of using Expando? Not sure if it's still problematic, but there was this: dart-lang/sdk#44333

An alternative would be to do something like this:

class PaintModel {
  final Paint paint;
  final List<double>? strokeDashPattern;
}

If it's not measurable, happy to merge as-is.

[blaugold]

I didn't benchmark it, but when testing manually I didn't notice any difference.

The last comment in the issue you linked shows benchmarks before and after the fix, from which I gather that the performance issue is resolved.

For what it's worth, I've used Expandos elsewhere and had no problems with performance.

[greensopinion]

web screenshot on #51

[greensopinion]

Thanks for the awesome contributions @blaugold !