Determine glyphs from default master

[madig]

Cosimo chat log

fontmake instantiator collects the list of glyph_names by unioning the font.keys of all the masters
it should rather take the default_font.keys() only
also, in collect_glyph_masters it silently skips if a glyph_name is not in source_layer, without checking if that source_layer is the default, in which case it should blow away
if a non-default master has a glyph which the default doesn't have, then that glyph should be ignored by definition