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.

Change-Id: Iba07070baafb5b802fc6da200696611afd2010d7