Merge branch 'splitscreen-parameter' into 'next'

Add `-splitscreen` commandline parameter

See merge request STJr/SRB2!1175

src/d_main.c

@@ -1589,6 +1589,12 @@ void D_SRB2Main(void)
 		ultimatemode = true;
 	}
 
+	if (M_CheckParm("-splitscreen"))
+	{
+		autostart = true;
+		splitscreen = true;
+	}
+
 	// rei/miru: bootmap (Idea: starts the game on a predefined map)
 	if (bootmap && !(M_CheckParm("-warp") && M_IsNextParm()))
 	{