Free editor splash image after splash screen

Since it's only used once, there's no need to keep it around.

src/scenedit/scen.graphics.cpp

@@ -373,6 +373,8 @@ void run_startup_g() {
 			break;
 	}
 	
+	// It's never needed again, so don't keep it in GPU memory
+	ResMgr::free<ImageRsrc>("edsplash");
 }
 
 void load_graphics(){