Add regions and peaks to designspace.

[bungeman]

Outline all the regions and their peaks for the active axes in the designspace view.

This was written to make it easier to see overlapping intermediate regions, though it is also somewhat useful for simpler variable regions as well.

[Lorp]

Thanks for this 👍 It’s very much appreciated. However I actually have a colour visualization of Regions in the Designspace panel coming soon. It will work a bit differently, so I won’t merge this one.

[bungeman]

Sounds good, just didn't want this to get lost (I wrote this mostly so I could get a better feel for what my own mechanically created fonts were actually doing). Feel free to use anything that looks interesting from this. Good to hear that something like it is in the works.

What I learned from writing this

1. the edges of intermediate regions tend to fall in the same place so the outlines tend to overlap, obscuring the edges. So should probably stroke only the inside edges of the rects to avoid overlap (not implemented in this PR).

2. it's really nice to know the positions of the peaks and which are active. This makes it really obvious that as one moves toward a point there will be more delta from that point.

3. highlighting the active regions makes it really obvious where the deltas for an active peak will drop off to zero. However when many active regions overlap one wishes for more of a diagonal hatch of their colors than the muddy brown one eventually ends up with using opacity. This is probably rather expensive and difficult to achieve.

[Lorp]

See issue #62