Frank Praznik
Add a gamepad mapping entry for Bluetooth DualShock 4 controllers on Linux.
The button mapping is the same as the USB controller, but the GUID is
different.
It was simpler to just have the polling (actually: hotplug detection)
functions return immediately if it's not an appropriate time to poll.
Note that previously, if any joystick/controller was opened, we would poll
every time anyhow, skipping this function.
This allows the joystick hotplug to function without the main event loop
(specifically: without SDL_INIT_VIDEO), and moves explicit polling for
joysticks where it belongs at the low-level: in SDL_SYS_JoystickDetect().
This lets apps call SDL_JoystickUpdate() to get hotplug events and keep
SDL_NumJoysticks() correct, as expected. As SDL_PumpEvents() (and
SDL_PollEvents, etc) calls SDL_JoystickUpdate(), existing apps will function
as before.
Thanks to "raskie" on the forums for pointing this out!
