Mac: Fix relative mode message after gaining focus.

This fixes a bug where relative mode would give a large jump if the
cursor was moved when the app doesn't have focus.

--HG--
extra : histedit_source : ff1b20a9e7f6e079c073c4ef761566b6c80b0f22%2C9879c6c838c69afea4aa2be80cbe137054600d12

src/events/SDL_mouse_c.h

@@ -73,6 +73,9 @@ typedef struct
     SDL_Cursor *def_cursor;
     SDL_Cursor *cur_cursor;
     SDL_bool cursor_shown;
+
+    /* Driver-dependent data. */
+    void *driverdata;
 } SDL_Mouse;