Added SDL_JoystickGetAxisInitialState() to get a joystick axis' initial value.

This is useful for controller mapping programs to determine an axis' zero state

src/joystick/SDL_sysjoystick.h

@@ -31,7 +31,8 @@
 /* The SDL joystick structure */
 typedef struct _SDL_JoystickAxisInfo
 {
-    Sint16 value;               /* Current axis states */
+    Sint16 initial_value;       /* Initial axis state */
+    Sint16 value;               /* Current axis state */
     Sint16 zero;                /* Zero point on the axis (-32768 for triggers) */
     SDL_bool has_initial_value; /* Whether we've seen a value on the axis yet */
     SDL_bool sent_initial_value; /* Whether we've sent the initial axis value */