Fixed walking animation; D-pad always on top

Full walking animation is now four frames, with the middle frame an alias of the starting frame. WorldScene control bar (including D-pad) is now drawn over the map instead of under. Initially touching the middle of the D-pad will still enable the D-pad when the finger is held on the screen and moved. Reorganized sprites into directories that make logical sense. Updated pack.json so that it combines all directories under assets gen\images\sprites into one spritesheet rather than separate images.