As that's the unit we use everywhere else in the LOK API. Also, make the
ScGlobal::nScreenPPTX/Y calculation more precise, otherwise rounding
errors occur during the pixel -> twip conversion.

Example with the old precision: col height is 103 px, nScreenPPTY is
0.067, twips is 1537.3134328358208, convering it back is 102.487562189
px.

Example with the new precision: col height is 103 px, nScreenPPTY is
0.0667, twips is 1544.2278860569716, convering it back is 102.948525737
px.

Change-Id: I19f5285508ef0c751614d07969b3a7a037e7d1ec