Added SDL_JoystickFromInstanceID() and SDL_GameControllerFromInstanceID().

src/joystick/SDL_joystick.c

@@ -373,6 +373,23 @@ SDL_JoystickInstanceID(SDL_Joystick * joystick)
     return (joystick->instance_id);
 }
 
+/*
+ * Find the SDL_Joystick that owns this instance id
+ */
+SDL_Joystick *
+SDL_JoystickFromInstanceID(SDL_JoystickID joyid)
+{
+    SDL_Joystick *joystick = SDL_joysticks;
+    while (joystick) {
+        if (joystick->instance_id == joyid) {
+            return joystick;
+        }
+        joystick = joystick->next;
+    }
+
+    return NULL;
+}
+
 /*
  * Get the friendly name of this joystick
  */