Split controller axes into positive and negative sides so each can be bound independently.

Using this a D-Pad can be mapped to a thumbstick and vice versa.
Also added support for inverted axes, improving trigger binding support

include/SDL_joystick.h

@@ -232,6 +232,8 @@ extern DECLSPEC void SDLCALL SDL_JoystickUpdate(void);
  */
 extern DECLSPEC int SDLCALL SDL_JoystickEventState(int state);
 
+#define SDL_JOYSTICK_AXIS_MAX   32767
+#define SDL_JOYSTICK_AXIS_MIN   -32768
 /**
  *  Get the current state of an axis control on a joystick.
  *