Fix BorderSide rendering

[HansMuller]

Fixes #7672

[Hixie]

How was your experience with paints?

[HansMuller]

vis paints: I was able to make a proper regression test without any trouble. However I wanted to overachieve and add tests for Borders with 2-4 different color sides.

However to do that I'd have to bake the order that the sides are painted into the test.

expect(find.byKey(key), paints
  ..path(color: red, style: PaintingStyle.fill) // top
  ..path(color: blue, style: PaintingStyle.fill) // right
  ..path(color: green, style: PaintingStyle.fill) // bottom
  ..path(color: orange, style: PaintingStyle.fill)); // left

Wiring the test to the current implementation didn't seem like a good idea, so I didn't.

I suppose if paints.path(color: red, style: PaintingStyle.fill) was true (maybe paints.contains.path...) if the display-list contained one such path element, then I could have written an expect statement for each border side.

[Hixie]

Cool, thanks for the feedback.