So that for large documents it's not needed to query all of them on
load, but (similar to tiled rendering itself) it's possible to query the
data that affects the visible area.

One catch is that the row sizes are relative, so there is a placeholder
row in case the visible area is not the top left corner, and
constructing its size needs special care. Normally the handed out twip
values have to be floored after twip->px conversion, but this one is
already rounded (as the total is a sum of px values, again becase of the
previous floor rule), so need to play the +0.5 trick to allow clients
always just flooring the logic conversion result they get.

(cherry picked from commit 75303695)

Conflicts:
	libreofficekit/qa/gtktiledviewer/gtktiledviewer.cxx
	sc/inc/docuno.hxx

Change-Id: I64a155582acdee7b2acc741d77a2c462409b91a8