Fix aVerticalLeftToRightBottomToTop:

- fnXDiff takes "logically larger, logically smaller" arguments, so it's
  first - second for top to bottom text (existing directions) but it's
  second - first for btlr

- fnXInc was effectively unused before, but it probably meant to provide
  logical left + width, so set that to physical left - width for btlr

And then port 3 places in lcl_UpDown() to use aRectFnSet, so we get
correct behavior for both ttb and btt directions.

With this, pressing the left/right arrow jumps to the previous / next
paragraph correctly.

Change-Id: I1323e95e2ce0ab2d66a3ea6e02bd50df2cebf232
Reviewed-on: https://gerrit.libreoffice.org/68462
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>