In ufo2ft preProcessor, we need to know which specific glyphs were actually modified (converted to quadratic), in order to do other things while processing filters, not simply if the fonts were modified as a whole; thus, here I changed fonts_to_quadratic to return the set of modified glyph names instead of just True/False. The change is backward compatible because code that checks whether the returned value is True/False will continue to work since bool(set) is True for non-empty set, False for empty ones.
