updating polygon sm to pixel conversion method, to make polygon processing more flexible for shapes other than squares.
fixing poly_coords.append() bug.
adding support for rounded_pixels option in layer_config
changing from rounded pixel to round_pixel (using centroid to remove the minus_poly creation, reducing overall processing)
removed unused "ignore_cache" option.
Expected that caching is done at a higher level, and not here.  (if cached properly get_tile() should not be called).
removing docstring mention of ignore_cache option.