tdf#84294: vcl: fix PDF export of transparent Writer frames in LO 4.3

The transparent frame background is painted opaque because
OutputDevice::DrawTransparent() records a MetaTransparentAction
instead of returning early.

Note that master and 4.4 use drawinglayer to paint backgrounds
so the problem is only visible in 4.3.

(regression from 36b59f2efc2bfb2c9256c39eb5687808deb)

Change-Id: Ide7a076b72123097a9fe099b96d36cda7f7bb082

vcl/source/outdev/transparent.cxx

@@ -611,6 +611,7 @@ void OutputDevice::DrawTransparent( const tools::PolyPolygon& rPolyPoly,
     if( !mbFillColor || (nTransparencePercent >= 100) )
     {
         DrawInvisiblePolygon( rPolyPoly );
+        return; // tdf#84294: do not record it in metafile
     }
 
     // handle metafile recording