The method assumed that the map mode of the output device is twips, but
that's not true for Impress. If paintTile() does the twips -> mm100
conversion, then LogicInvalidate() has to do the reverse of that.

This fixes typing into the main placeholder shape of an empty Impress
document: with our 256x256px tiles only the 3rd and 4th row was
repainted previously, while the interesting content is in the 2nd row.

Change-Id: I8bda113e73335e465c1c4626a58b6be2b232a190