Instead of in PaintHelper::StartBufferedPaint(). If the buffer size is
set to match the size of a sub-widget (for which the paint was
triggered), then client using the buffer as a persistent vdev are unable
to paint at arbitrary locations.

For example, if we painted the ruler, then the blinking cursor won't be
able to paint to the SwEditWin area.

(cherry picked from commits 161f7533 and
43ac95ab)

Change-Id: Iba07070baafb5b802fc6da200696611afd2010d7
Reviewed-on: https://gerrit.libreoffice.org/17490Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>