Adds "Clip Region" clipping to ConsoleDriver

[tig]

As part of the new View architecture, we will need a more sophisticated clipping mechanism than just a single rectangle.

This PR adds such a thing, retaining the previous ConsoleDriver.Clip functionality.

The clip region is exposed via ClipRegion which is a List<Rect>. IsInClipRegion() (private) tests whether a coordinate is within the region or not.

ConsoleDriver.IsValidClip has been renamed to IsValidLocation (it was mis-named before because it also validates the coordinates are within the screen bounds).

Also:

• cleans up ConsoleDriver code

Other than ensuring I didn't break anything and adding unit tests, I've not exercised this functionality for realz yet. That will come soon, hence this is still a draft PR.

[tig]

Still a draft.

I've discovered a bunch of things in all the drivers that need to be tweaked to make this work. No way am I going to do it all 4 times. Instead, I'm going to work on #666 (which I started in the code in this PR) as a separate PR first. It's just crazy how much duplicate/similar code there are in all 4 drivers. Much can be moved into ConsoleDriver in a way that makes unit testing it all much better.