Check for compatibility with cu2qu rather than with FontParts

[thundernixon]

Right now, VarFont Prep misses a bunch of glyphs, because it relies on the FontParts check isCompatible(), which isn't as strict as the compatibility requirements in cu2qu (the converter to TTF/quadratic curves).

I want to change this, so more glyphs are removed as needed.

[arrowtype]

Note to self: maybe the script could just do the actual conversion, glyph be glyph, perhaps in a "try, except" setup. try converting, except compatibility error, remove glyph