valgrind: memory leak in scoped_ptr/unique_ptr -> VclPtr convert

regression from

commit 820576af
Author: Noel Grandin <noel@peralex.com>
Date:   Thu Mar 19 13:54:12 2015 +0200

    start wrapping OutputDevice in VclPtr

 9,800 (568 direct, 9,232 indirect) bytes in 1 blocks are definitely lost in loss record 12,696 of 12,898
    by 0xC602E72: ScopedVclPtrInstance<VirtualDevice>::ScopedVclPtrInstance<>() (vclptr.hxx:375)

(cherry picked from commit 23d4d2f6)

Change-Id: I356f39c339fd28a9e19a00d61b6f1bee492f2638
Reviewed-on: https://gerrit.libreoffice.org/18689Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>

vcl/source/outdev/transparent.cxx

@@ -751,7 +751,7 @@ void OutputDevice::DrawTransparent( const GDIMetaFile& rMtf, const Point& rPos,
 
                     const AlphaMask aAlpha(xVDev->GetBitmap(aPoint, xVDev->GetOutputSizePixel()));
 
-                    xVDev.reset();
+                    xVDev.disposeAndClear();
 
                     // draw masked content to target and restore MapMode
                     DrawBitmapEx(aDstRect.TopLeft(), BitmapEx(aPaint, aAlpha));
@@ -799,7 +799,7 @@ void OutputDevice::DrawTransparent( const GDIMetaFile& rMtf, const Point& rPos,
 
                     aAlpha = xVDev->GetBitmap( Point(), xVDev->GetOutputSizePixel() );
 
-                    xVDev.reset();
+                    xVDev.disposeAndClear();
 
                     EnableMapMode( false );
                     DrawBitmapEx( aDstRect.TopLeft(), BitmapEx( aPaint, aAlpha ) );