fix for composer map item's background when non-opaque

src/core/composer/qgscomposermap.cpp

@@ -227,7 +227,8 @@ void QgsComposerMap::cache( void )
   double forcedWidthScaleFactor = w / requestExtent.width() / mapUnitsToMM();
 
   mCacheImage = QImage( w, h,  QImage::Format_ARGB32 );
-  mCacheImage.fill( brush().color().rgb() ); //consider the item background brush
+  mCacheImage.fill( QColor(255,255,255,0).rgba() ); // the background is drawn by composerItem, but we still need to start with that empty fill to avoid artifacts
+
   double mapUnitsPerPixel = mExtent.width() / w;
 
   // WARNING: ymax in QgsMapToPixel is device height!!!