Added Steam Input API support for game controllers

Added support for getting the real controller info, as well as the function SDL_GameControllerGetSteamHandle() to get the Steam Input API handle, from the virtual gamepads provided by Steam.

Also added an event SDL_CONTROLLERSTEAMHANDLEUPDATED which is triggered when a controller's API handle changes, e.g. the controllers were reassigned slots in the Steam UI.

(cherry picked from commit c981a597dc)

include/SDL_events.h

@@ -131,6 +131,8 @@ typedef enum
     SDL_CONTROLLERTOUCHPADMOTION,      /**< Game controller touchpad finger was moved */
     SDL_CONTROLLERTOUCHPADUP,          /**< Game controller touchpad finger was lifted */
     SDL_CONTROLLERSENSORUPDATE,        /**< Game controller sensor was updated */
+    SDL_CONTROLLERUPDATECOMPLETE_RESERVED_FOR_SDL3,
+    SDL_CONTROLLERSTEAMHANDLEUPDATED,  /**< Game controller Steam handle has changed */
 
     /* Touch events */
     SDL_FINGERDOWN      = 0x700,
@@ -446,7 +448,7 @@ typedef struct SDL_ControllerButtonEvent
  */
 typedef struct SDL_ControllerDeviceEvent
 {
-    Uint32 type;        /**< ::SDL_CONTROLLERDEVICEADDED, ::SDL_CONTROLLERDEVICEREMOVED, or ::SDL_CONTROLLERDEVICEREMAPPED */
+    Uint32 type;        /**< ::SDL_CONTROLLERDEVICEADDED, ::SDL_CONTROLLERDEVICEREMOVED, ::SDL_CONTROLLERDEVICEREMAPPED, or ::SDL_CONTROLLERSTEAMHANDLEUPDATED */
     Uint32 timestamp;   /**< In milliseconds, populated using SDL_GetTicks() */
     Sint32 which;       /**< The joystick device index for the ADDED event, instance id for the REMOVED or REMAPPED event */
 } SDL_ControllerDeviceEvent;