Simplest way to draw a line across a screen

This patch uses a jit.matrix to move a line across the screen by first using the setall message to set all the cells of the jit.matrix to 255 which causes all pixels in the display window to be set to white. Next the counter object sets a cell of the jit.matrix determined by the counter object to 0 which causes the corresponding column of pixels in the display window to turn black.