feat: per-side border widths (#836)

* feat: per-side border widths

* refactor: make `BorderWidth::is_all_zero` more sane

* fix: wrong operator in `is_all_zero`

* implement parsing for per-side widths

* feat: impl `Display` for `Border`

* fix lints

* fix another lint

* chore: fmt

* fix tests

* fix: add `RRect::rect` to mocked engine

* chore: improve border test coverage

* fix: border test assertion

* fix: radius offset calculations for different border alignments

* clean up border drawing code

* fmt

* revert needless `get_rounded_rect` name change

* format

* add `PathFillType` to mocked engine

* mock `Path::set_fill_type`

* update docs

* fix jagged antialiasing on borders

* lint

* mock `Canvas::draw_drrect` and `RRect::bounds`

* simplify border visibility check

Co-authored-by: Marc Espin <[email protected]>

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Co-authored-by: Marc Espin <[email protected]>