Clear sprites to white fdo#45219.

Another fix for a rendering glitch mentioned in fdo#45219 -
vclcanvas sadly does not properly implement compositing, so all-
black background yields dark shadows on semi-transparent objects.

Ultimately though, we should retire vclcanvas instead.

slideshow/source/engine/slideview.cxx

@@ -206,7 +206,7 @@ void clearRect( ::cppcanvas::CanvasSharedPtr const& pCanvas,
     if( pPolyPoly )
     {
         pPolyPoly->setCompositeOp( cppcanvas::CanvasGraphic::SOURCE );
-        pPolyPoly->setRGBAFillColor( 0x00000000U );
+        pPolyPoly->setRGBAFillColor( 0xFFFFFF00U );
         pPolyPoly->draw();
     }