With this, code that wants to take a persistent render context (e.g.
vcl::Cursor) will be able to do so. This commit just moves the buffer,
though: it's still created / deleted by PaintHelper.

Having it in ImplFrameData means that we'll have one buffer / one
system window: i.e. toplevel window and its sub-widgets share a buffer.

(cherry picked from commit a33e0379)

Conflicts:
	vcl/source/window/paint.cxx

Change-Id: Ic37220a2a483a7389aa04cd4313b6fc61a5408bf
Reviewed-on: https://gerrit.libreoffice.org/17414Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>