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.
This better harmonizes fallback fonts with the primary font by matching
the heights of lowercase letters. This should be a big improvement for
users who use mixed scripts and so rely heavily on fallback fonts.
This sets the stage for dynamically adjusting the sizes of fallback
fonts based on the primary font's face metrics. It also removes a lot of
unnecessary work when loading fallback fonts, since we only actually use
the metrics based on the parimary font.
