CanvasOpen ( n, w, h, viewport_x, viewport_y, viewport_w, viewport_h, mode)

Opens a canvas for drawing.

n - the canvas index; there is a max of 8 ( 0 to 7 )

w, h - the total size of the drawing surface

viewport_x, viewport_y - where on the window to draw the canvas
viewport_w, viewport_h - size of the canvas area to be shown

Mode 0 - Solid
Mode 1 - Transparent

Example:
    CanvasOpen ( 0, 640, 480, 0, 0, 640, 480, 1 )