Fix possible issue for transparency in landscapes (see #3512)

src/core/modules/LandscapeMgr.hpp

@@ -576,11 +576,10 @@ public slots:
 			flagLandscapeUseTransparency=b;
 			emit flagLandscapeUseTransparencyChanged(b);
 		}
-		if (b==false)
+		if (!flagLandscapeUseTransparency) // equals to false
 			landscape->setTransparency(0.0);
 	}
 
-
 	/*
 	//This method has been removed, use StelSkyDrawer::getBortleScaleIndex instead, or StelMainScriptAPI::getBortleScaleIndex in scripts
 	//Also, if required, please use StelSkyDrawer::setBortleScaleIndex or StelMainScriptAPI::setBortleScaleIndex instead of LandscapeMgr::setAtmosphereBortleLightPollution