Use cylindrical placement for key legends

[jepler]

I noticed that when placing legends on cherry keycaps, the depth varies across the key. This is because the text is placed on a plane, not along the curve of the top key surface.

I'm printing resin keycaps then adding color legends using gel nail polish; I think that having consistent depth of the inset legends will lead to better results.

I investigated options to correct this. One possibility is the openscad "text on" library. Of course, this library comes with its own set of caveats, many resulting from the fact that it's not possible to directly measure text metrics from within scad code.

Here's the result of my experimentation. I don't think it's suitable to merge in its current state, so please consider this a possible starting point for discussion.

This copies in the text_on library, which I believe is licensed compatibly (LGPL 2.1 upgrades to GPL2-or-later, which in turn upgrades to GPL3).

Please feel free to close this up if it's not a direction you're interested in pursuing. In any case, thanks for KeyV2.