Side bearing display/update

[raphlinus]

A common font editor feature is to report "side bearings", which is the inset from the advance to the bounding box. To display these, use the kurbo bounding box feature on the path. Lens update is slightly trickier. The right side bearing functions much as lensing the advance, but updating the left side bearing does a nudge of all points in the glyph, as well as the advance.

This would be added to the current advance display in sidebar, but I think it would also be useful to have a metrics mini-panel on the same row as the coord pane. That could be a separate issue though, as it's UX polish rather than core functionality.

I should also point out that doing fitting using side bearings "by the number" is a bit of an antipattern, but that's just for information. An extremely common use case is "this needs 10 more units on the left".

[davelab6]

It seems this is now mostly implemented, although I found an odd bug, where after editing the advance width, editing the sidebearings works, but not immediately.

https://youtu.be/aonLkcQJGTQ

[cmyr]

Ah yea, this is an issue where these initial placeholder glyphs start with zero advance width, and the sidebearing editing doesn't work when there's no advance set. I'll open a separate issue for that.