source and dest are two canvas
ids of the source canvas (to copy from) and the destination canvas
(to copy to). If no other parameters are specifed then just a
direct copy from one canvas to another is performed.
x1,y1 specfies the top left coordinate of the rectangle
to copy from the source.
x2,y2 specfies the bottom right coordinate of the rectangle
to copy from the source.
destx and desty are the co-ordinates in the
destination canvas of the top,left corner of the copy rect.