A lot of software gets confused when there are named instances that differ
only by opsz. This change removes all "Display" instances from the fvar table
and makes opsz=32 the default, so that software without automatic opsz-to-size
mapping displays the "Display" styles instead of the text styles by default.
This is the same approach taken by Apple San Francisco Pro.
Closes#519
This removes the slant/italic variable axis and breaks up the font in two: roman and italic. This change will allow diverging designs for italic (for example single-storey a). It also addresses the fact that most software, including web browsers, doesn't handle VFs with slnt or ital well.
