I noticed that there was an off-by-one error in cell height adjustment
when the number of pixels to add/subtract is odd. The metrics measured
from the top would be shifted by one less than they should, so, for
example, the underline position would move one pixel closer to the
baseline than it had been (or one pixel further away if subtracting).

Also noticed that the overline position was missing here, so added that.

This reverts commit 23cc50b12c1b670ff3f96c63437f61742d3b4d3c.

Move size adjustment logic out of `Entry`, I understand the impulse to
put it there but it results in passing a lot of stuff around which isn't
great.

Rework `add(...)` in to `add(...)` and `addDeferred(...)`, faces are
passed directly now instead of passing an entry, and an options struct
is used instead of positional arguments for things like style, fallback,
and size adjustment.

Change size adjustment test back to a half pixel tolerance instead of 5%
because the previous commit (allowing fractional pixel sizes) fixed the
root cause of large differences.

Icons were often WAY too big before because they were filling the whole
cell in height, which isn't great lol. This commit adds an `icon_height`
metric which is used to constrain glyphs that shouldn't be the size of
the entire cell.