if not all masters have .notdef, building VF can fail if generated .notdef is incompatible

[anthrotype]

ufo2ft always add a .notdef glyph, even for sparse layers. Often the main masters have .notdef already, but the sparse layers do not, and ufo2ft adds one (by using the font metrics to draw the rectangle). However, this generated .notdef may not be interpolation-compatible with the existing .notdef glyphs in the other masters.
This can lead to varLib errors when generating a variable font. I noticed that CFF2 variable output is more likely to fail with an error, whereas for TT-flavored VF it passes silently (the two nested rectangles happen to be interpolatable but the interpolation still is incorrect, as they may swap place, etc.).

We probably should not add a .notdef glyph for sparse layers sort of masters.

[anthrotype]

We probably should not add a .notdef glyph for sparse layers sort of masters.

actually, we cannot not add a .notdef, glyphID 0 must always be .notdef (plus fonttools doesn't let us save CFF font without .notdef), so we have to add a compatible .notdef glyph to all the sparse masters if they do not have one already, which is what #387 does. We take the .notdef from the default base master and copy it over to all the other masters that do not already have one (which is mostly the sparse ones).