In the paints, we must use the size of the Window for the computations, not of
the RenderContext - the RenderContext can be much bigger than the Window in
the double-buffering case.

Fixes for example the list boxes, and many others.

Change-Id: I4c7607569f88b2d097587140858d0862e54b5ea6
Reviewed-on: https://gerrit.libreoffice.org/16421Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Adolfo Jayme Barrientos <fitojb@ubuntu.com>