tdf#92982 vcl rendercontext: move buffer from PaintHelper to ImplFrameData

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.

Change-Id: Ic37220a2a483a7389aa04cd4313b6fc61a5408bf

include/vcl/window.hxx

@@ -729,11 +729,8 @@ private:
         This function is kind of recursive - it may be called from the
         PaintHelper destructor; and on the other hand it creates PaintHelper
         that (when destructed) calls other ImplCallPaint()'s.
-
-        @param rBuffer VirtualDevice for double-buffering.  It is only passed
-        here, the actual handling happens in the PaintHelper.
     */
-    SAL_DLLPRIVATE void                 ImplCallPaint(const VclPtr<VirtualDevice>& rBuffer, const vcl::Region* pRegion, sal_uInt16 nPaintFlags);
+    SAL_DLLPRIVATE void                 ImplCallPaint(const vcl::Region* pRegion, sal_uInt16 nPaintFlags);
 
     SAL_DLLPRIVATE void                 ImplCallOverlapPaint();
     SAL_DLLPRIVATE void                 ImplPostPaint();

vcl/inc/window.h

@@ -179,6 +179,7 @@ struct ImplFrameData
     ::com::sun::star::uno::Reference< ::com::sun::star::datatransfer::clipboard::XClipboard > mxSelection;
 
     bool                mbInternalDragGestureRecognizer;
+    VclPtr<VirtualDevice> mpBuffer; ///< Buffer for the double-buffering
 };
 
 struct ImplAccessibleInfos