Use oox::drawingml::convertEmuToHmm()

And get rid of the writerfilter copy, that does no rounding.  Adjusted
testcases:

- testFdo80555: 245 -> 247 (should be 246.944444444, so a good change)
- testDMLGroupShapeChildPosition: roundtripped values are now closer to
  the initial ones, so also a good change

Change-Id: I4dec7857a0df77face01b7a8ba1da7c647a24b6c