DrawBackground

Hi Krisztian,

I've pushed your DrawBackground patch, together with a small follow-up
patch that fixes the overlaps.  The follow-up patch is this:

http://cgit.freedesktop.org/libreoffice/core/commit/?id=7c927b64fa42ea1b0ddf97ce6b6aa400f2e30d5b

I've extended the documentation of the bWorksInPixels, please check it,
hopefully it is now even more understandable what is the issue there,
and where the confusion comes from when we are painting to the
screen :-)

Also now I know where the bMetaFile comes into play.  When we were using
that in DrawBackground, it was breaking the generation of thumbnails in
the startcenter - so that is where it is used.  Given that, now I am a
bit unsure if we should use that in ScOutputData::DrawGrid at all; but
so far I don't see any harm in doing that; and it was there originally,
so...

To summarize all the test cases that need checking for every change
there:

* normal painting to the screen
* PDF export
* bin/run gtktiledviewer /local/libreoffice/master/instdir/program <file>
* thumbnail in the startcenter

Hope that helps!

All the best,
Kendy