[WIP] SDL2 on macOS detects joysticks only with SDL2_INIT_VIDEO

[EliaCereda]

SDL2 on macOS seems to have issues detecting joysticks that are already connected at startup (I started a discussion here: libsdl-org/SDL#12886).

I'm opening a PR to share this workaround that seems to make everything work correctly, but it's not a proper fix right now. I'm sure initialising SDL2_INIT_VIDEO is not very desirable, especially not on other platforms, as it prevents the joy node from running headless.

Was this a known issue?

[peci1]

This PR seems to contain a lot of unrelated changes. Or are they related?

I think this workaround has to be enabled by a parameter (that defaults to false).

[EliaCereda]

Sorry for not noticing your reply earlier @peci1.

Ideally SDL would fix this upstream, by properly setting up the macOS event loop even without SDL2_INIT_VIDEO. At the moment my issue there is still open:
libsdl-org/SDL#12886

In the meantime, my changes are indeed related: in joy.cpp, once you enable SDL2_INIT_VIDEO, the change to eventThread is needed to call SDL_WaitEventTimeout from the main thread. That's because macOS GUI APIs can only be invoked from the main thread. IIRC they would throw an assertion without the change.

The changes to joy_enumerate_devices.cpp on the other hand are needed because joysticks are discovered asynchronously on macOS. If you just enumerate them at startup you would get an empty loop.