[Simh] Adding USB game controller support

[Lars Brinkhoff]

Hello,

I'm adding USB game controller support to SIMH.  My primary motivation
is simulation of historical devices like Spacewar consoles, joysticks,
buttons, etc.  I have updated the PDP-6/KA10 simulator to use this, and
I'm planning to do PDP-1 (Spacewar) and PDP-11 (Dazzle Dart) next.

I'm building on the APIs provided by the SDL2 library.  In this API, a
game controller reports events for some number of axes and buttons.  An
axis is an analog value, typically for a lever.  Each joystick on a
device has two axes.

The API I added to sim_video.h looks like this:

   typedef void (*VID_GAMEPAD_CALLBACK)(int, int, int);
   t_stat vid_register_gamepad_motion_callback (VID_GAMEPAD_CALLBACK);
   t_stat vid_register_gamepad_button_callback (VID_GAMEPAD_CALLBACK);

A SIMH simulator must first call vid_open, and can then register to
receive game controller events.  When vid_close is called, all callbacks
are unregistered.  More than one callback can be registered; the maximum
is currently 10.

When a game controller reports an event, the corresponding callback will
be called with three values.  For a motion event, these are:

   1. The device ID.
   2. The axis on this device.
   3. The value, from -32768 to 32767.

For a button event:

   1. The device ID.
   2. The button on this device.
   3. The state, 0 for released and 1 for pressed.

The device ID, axis, and button are all enumerated starting from 0.