Screen jumping when keyboard controls (arrows) used.

I noticed that the screen will jump when you use the keyboard arrows to control piece movement. This is because the captured command is passed through-fine for using letter keys but the arrows are also used for navigation.
I fixed this by adding a 'return false' after line 1112 of the tetris.js file (end of function tetris_press_key(e, evento_actual) )

1112: tetris_show_map(map_matrix, map_matrix_previous);
return false; /* return false to prevent default event */

I've attached a patch.