[glyf] enable OVERLAP_COMPOUND composite flags
[glyf] enable OVERLAP_COMPOUND composite flags

it might be deprecated or ignored in some rasterizers, but I don't
see why we should discard it, if it's present in the input font.

This also allows to set the flag, which may turn out to be useful
in some circumstances.

cf. https://github.com/googlei18n/fontmake/issues/253#issuecomment-335600887

googlefonts/fonttools,fonttools/fonttools