Initialize interface structures so they can be extended in the future

We guarantee that we will only add to the end of these interfaces, and any new fields will be optional.

include/SDL3/SDL_joystick.h

@@ -414,16 +414,18 @@ typedef struct SDL_VirtualJoystickSensorDesc
 /**
  * The structure that describes a virtual joystick.
  *
- * All elements of this structure are optional and can be left 0.
+ * This structure should be initialized using SDL_INIT_INTERFACE(). All elements of this structure are optional.
  *
  * \since This struct is available since SDL 3.0.0.
  *
  * \sa SDL_AttachVirtualJoystick
+ * \sa SDL_INIT_INTERFACE
  * \sa SDL_VirtualJoystickSensorDesc
  * \sa SDL_VirtualJoystickTouchpadDesc
  */
 typedef struct SDL_VirtualJoystickDesc
 {
+    Uint32 version;     /**< the version of this interface */
     Uint16 type;        /**< `SDL_JoystickType` */
     Uint16 padding;     /**< unused */
     Uint16 vendor_id;   /**< the USB vendor ID of this joystick */
@@ -454,10 +456,20 @@ typedef struct SDL_VirtualJoystickDesc
     void (SDLCALL *Cleanup)(void *userdata); /**< Cleans up the userdata when the joystick is detached */
 } SDL_VirtualJoystickDesc;
 
+/* Check the size of SDL_VirtualJoystickDesc
+ *
+ * If this assert fails, either the compiler is padding to an unexpected size,
+ * or the interface has been updated and this should be updated to match and
+ * the code using this interface should be updated to handle the old version.
+ */
+SDL_COMPILE_TIME_ASSERT(SDL_VirtualJoystickDesc_SIZE,
+    (sizeof(void *) == 4 && sizeof(SDL_VirtualJoystickDesc) == 84) ||
+    (sizeof(void *) == 8 && sizeof(SDL_VirtualJoystickDesc) == 136));
+
 /**
  * Attach a new virtual joystick.
  *
- * \param desc joystick description.
+ * \param desc joystick description, initialized using SDL_INIT_INTERFACE().
  * \returns the joystick instance ID, or 0 on failure; call SDL_GetError() for
  *          more information.
  *