This enables selecting an italic font when the cell is italic,
but has more power beyond just that simple property.
This runs a little hot on the CPU so there's probably some caching and
tweaking that can be done to make the evaluation a bit cheaper.
So that we no longer assume Operator Mono SSm.
We now default to `monospace`. I just tried this and the font metrics
appear to be wonky, so there's some tweaking to do.
