- Increase the threshold for "dark" colours from Luminosity<=38 to
  Luminosity<=60.

  This makes LibreOffice align more closely with MS Word when
  considering whether "automatic" font colour should be black or white,
  a decision that is based on whether the background colour is "dark".

  Beware that this also affects other areas.

- Fix the coefficients for GetLuminosity which appeared to be off
  by one according to the "Rec 601" coefficients for Luma quoted at
  http://en.wikipedia.org/wiki/Luma_%28video%29

Change-Id: I0af1f77909713e3db5ea8ee98456146569679594
Reviewed-on: https://gerrit.libreoffice.org/2601Reviewed-by: Fridrich Strba <fridrich@documentfoundation.org>
Tested-by: Fridrich Strba <fridrich@documentfoundation.org>