== DETAILS
Starting to implement the I/O handling on the HID driver.
The old implementation basically had a never-ending HIDRead() callback
set up, so that callback N would start the read process for invocation
N+1.
We will take the same approach here; but now that the I/O thread is
happenning on its own core, we should be able to let it run full-
throttle without impacting emulator performance.
Of course, this hinges on the callback actually running on the same
core as the syscall was initiated on.
== TESTING
Can confirm that the read_loop_callback gets invoked on the same core
that the HIDRead() was invoked on.
Allow using the Gamepad's touch screen as a RETRO_DEVICE_POINTER.
Methodology could use some work, had to add an extra axis to
joypad in order to get the data transferred into the input driver.
Feel free to change this.
Needs to emulate RETRO_DEVICE_LIGHTGUN to really be useful.
Potential for Wiimote IR in future.
Partially addresses libretro/RetroArch#5294
