tdf#92982 vcl rendercontext: fix buffer size with empty user profile

I did not notice this before, as my user profile had a custom window
size; but with an empty user profile the buffer had a 0,0 size, so the
buffered result was empty, as no ImplHandleResize() was invoked.

Change-Id: Ie299ad1323944941afc407dc90f2459d72885d42
(cherry picked from commit 968bc55a)
Reviewed-on: https://gerrit.libreoffice.org/17532Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>

vcl/source/window/window.cxx

@@ -2481,6 +2481,10 @@ void Window::Show(bool bVisible, sal_uInt16 nFlags)
                 mpWindowImpl->mpFrame->GetClientSize( nOutWidth, nOutHeight );
                 ImplHandleResize( this, nOutWidth, nOutHeight );
             }
+
+            if (mpWindowImpl->mpFrameData->mpBuffer && mpWindowImpl->mpFrameData->mpBuffer->GetOutputSizePixel() != GetOutputSizePixel())
+                // Make sure that the buffer size matches the window size, even if no resize was needed.
+                mpWindowImpl->mpFrameData->mpBuffer->SetOutputSizePixel(GetOutputSizePixel());
         }
 
         if( aDogTag.IsDead() )