FPS down to 30 on Mac even in window mode

[Rubonnek]

• cocos2d-x version: latest git version up to commit a7482b4
• devices test on: Mac Mini
• developing environments
  • Xcode version: 8.3.2

Steps to Reproduce:

1. Create a new C++ project.
2. Compile and run a new C++ project.
3. The FPS stays at 30.

Note: also compiling cpp-tests will result in the same problem.

[minggo]

I think it is duplicated with #16561

[Rubonnek]

The problem I'm reporting is a bit different from #16561 since the FPS drop in this case happens even while not in fullscreen. I mean, I don't have to do anything at all after running the C++ project to get the FPS to stay at 30, but both of the problems do seem to be related. The cause could be the same.