font_project: make it clearer that only 'mixed' glyphs are decomposed in TTF
font_project: make it clearer that only 'mixed' glyphs are decomposed in TTF

I was a bit confused by the `glyph_filter: lambda g: g` until I figured out that a Defcon Glyph object evaluates to True if it contains any contours.

Apparently, that's how user-defined classes that implement __len__() work:
https://docs.python.org/3/library/stdtypes.html#truth-value-testing

googlefonts/fontmake,googlefonts/fontmake,googlei18n/fontmake,googlei18n/fontmake