VF compilation: All sparse layers need a .notdef if the normal masters have it

If a font comes with a custom `.notdef` glyph, it must be inserted into all sparse layers, too, because they are compiled into standalone fonts for later merging into a VF. ufo2ft will insert a standard `.notdef` if one isn't present in the sparse layers, which might be incompatible with the other designed ones.

Not sure what to do here, maybe warn the user? Only a problem for VF compilation due to the way it werks, instance generation is fine.