When you put a box in a Viewport2, it might paint outside its allocated bounds.
This patch teaches Viewport2 to clip when that happens.

I plan to use this to implement similar logic in SliverPadding.

To make this easier to test I extended the paints matcher to accept a
function that takes a canvas. While I was at it I also made it accept
a Finder, it'll go and find the render object for you.

Also added support for paints..path and fixed some grammar in the
error messages.

Also improved the docs for debugPaint*.

Teach `paints` matches about `drawRect` and how to `paintChild`.

Also, improve test coverage.

This will make it easier to understand what this file does.

MockCanvas to use it instead.

Currently it only supports a very limited set of drawing features, but
adding new ones is easy.

Once the feature set is more complete, and we've used this a bunch, if
it feels right, this API will get promoted to flutter_test.

This patch changes the default appearance of Slider to not have the
thumb be an open circle at its minimum position. The `thumbOpenAtMin`
property can enable drawing an open thumb at the min position, which was
the previous behavior.

Fixes #6941