Previously, the rows and columns arguments had different semantics. Now
they have the same semantics. The new API also uses Iterable rather than
List to give clients more flexiblity in how they construct these
arguments. For example, RenderTable no longer needs to reify the
reversed list of column positions.

Also, fix asserts in TableBorder#paint to match documentation.

Fixes #12009

This will enable both to be RTL'ed.

Also, factor out common border painting code into paintBorder.
Also, make Border paint uniform non-rounded borders using drawRect.
Also, add some documentation about an issue that wasted an hour of my life.
Also, factor out all the border painting code into TableBorder.paint.