[_a_v_a_r] normalize output of toXML() by roundtripping float->fixed->float
[_a_v_a_r] normalize output of toXML() by roundtripping float->fixed->float

Otherwise dumping avar table to TTX after it's been generated by
varLib could produce different float strings than when dumping the
same avar table after compiling and decompiling.

fonttools/fonttools,googlefonts/fonttools