In case we had a toplevel window W1, the paint was triggered for window
W2 and we had a sub-widget W3, then previously the buffer was created
for W2, so the pixel offsets had to be set relative to W2 when rendering
W3. As a consequence, if a single window was painted, then it was
always painted in the top left corner.

Now that the buffer is persistent and is always created for W1, make
sure that we paint to the correct offset, and W3 is always painted at
the same offset, regardless if it was painted directly, or just because
it's a child of W2.

With this, the buffer conents is closer to what is on the screen, even
if it's not perfect yet.

Also:

- PaintHelper: restore set buffer properties
- let PaintBufferGuard also manage pixel offset
- no need to tweak map mode in PaintBuffer()

Conflicts:
	vcl/source/window/paint.cxx

(cherry picked from commits 94a6daa0,
c36a00b8,
28e465d2 and
ec1a0354)

Change-Id: Ibf0e89ad18e5763bd2a01e69d91da163c24a309d
Reviewed-on: https://gerrit.libreoffice.org/17518Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>