silence a bunch of msvc warnings

src/StepMania.cpp

@@ -1628,8 +1628,8 @@ int LuaFunc_SaveScreenshot(lua_State *L)
 	// If pn is provided, save to that player's profile.
 	// Otherwise, save to the machine.
 	PlayerNumber pn= Enum::Check<PlayerNumber>(L, 1, true);
-	bool compress= lua_toboolean(L, 2);
-	bool sign= lua_toboolean(L, 3);
+	bool compress= lua_toboolean(L, 2) > 0;
+	bool sign= lua_toboolean(L, 3) > 0;
 	RString prefix= luaL_optstring(L, 4, "");
 	RString suffix= luaL_optstring(L, 5, "");
 	RString dir;