Trigger SDL_CONTROLLERDEVICEADDED for all gamepads already connected
when scrcpy starts. We want to handle both the gamepads initially
connected and the gamepads connected while scrcpy is running.
It is a bit racy, because a device may be added after the controller
subsystem initialization, but before the call to list the joysticks. In
that case, a single device may in theory be detected as added twice.
This should be harmless though (the second initialization will just fail
without major impact).
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