Adds some plumbing to track the current mouse position and
extract the hyperlink from the cell that is being hovered over.
We render those cells with underline, possibly in a different color.
We don't yet do anything on click.
This felt a bit repeatey and it pre-dated the TerminalHost trait.
I'd like to remove it completely but there are some frustrating and
fiddly lifetime concerns with mutable TerminalHost reference so I'm
hiding it from the public interface and bridging it the answerback
stream into the host at the bottom of the advance_bytes method for
now.
This also improves perf. The issue was that the erase of the whole
line of the background may be observed by the X server when SHM is
enabled because we don't wait for the exposure/copy to the window
pixmap before updating the same row fractionally later.
Avoiding the unconditional erase and just painting the full cell
contents over means that there's no opportunity for a visible flash.
In addition, since we render the cells background individually, that
erase was not needed.
This should save us some work and take some load off the cpu.
This wasn't correctly handling double-width or ligatures. In addition,
don't respect the x_advance outside of a ligature run, as it can result
in slightly offset columns for example when `ls -l` shows `--` and that
sequence has a slightly less wide ligatured rendition.
```
echo -e '\e[4mu--nder\e[0m \e[21mdo--uble\e[0m \e[9mst--rike\e[0m'
```
We were panicking when rendering the || ligature, which is narrower
than we were expecting. There was also a weird looking assert
that was triggering if we got path the math overflow.
