cocoa: fix fullscreen regression on 10.11 and newer

[Akemi]

after some tests this fixes #3364 and potentially fixes #2392 (on 10.12) or majorly improves the situation (10.11+).

a bundle can be found here for testing.

[pigoz]

Does this not cause any tearing? Why does it work better?

Generally using double buffering is a good idea, and the other backends do use it.

[ghost]

I suspect the difference doesn't really matter, because OSX is full composited. On the other hand, it must be doing something (just that it has a negative effect here).

From the experiments I had the impression that fullscreen does something similar to "unredirecting" a window on x11 compositors, i.e. giving it direct access to the driver's framebuffer. Maybe disabling double buffering makes the window always composited? (I'm just guessing here.)

[ghost]

Merged.