CanvasClip ( ImgSlot, src_x, src_y, src_w, src_h, flag )

Saves a portion of the current canvas to an image slot.

NOTE: THIS ROUTINE SHOULD NOT BE USED DURING
SPEED CRITICAL MOMENTS OF YOUR PROGRAM

Flag - If 0 then saved image will be accessible from any window at the expense of speed
If Flag is 1 then saved image will only be accessible in the window it was created

Example:
    CanvasClip ( 1, 0, 0, 50, 50, 1 )