Set default alpha value on ImageData:setPixel to 255. resolves #575

--HG--
branch : imagedata-alpha-default

src/modules/image/wrap_ImageData.cpp

@@ -85,7 +85,8 @@ int w_ImageData_setPixel(lua_State *L)
 	c.r = luaL_checkint(L, 4);
 	c.g = luaL_checkint(L, 5);
 	c.b = luaL_checkint(L, 6);
-	c.a = luaL_checkint(L, 7);
+	c.a = luaL_optint(L,7,255);
+	//c.a = luaL_checkint(L,7);
 	try
 	{
 		t->setPixel(x, y, c);