This will be used to implement painting of 2D paths. This first patch
adds support for line_to(), move_to() and close().
It will try to have the same semantics as the HTML <canvas> element.
To stroke a Path, simply pass it to Painter::stroke_path().
blit_filtered() can be used to easily implement per-pixel filtered blit
functions. All you need to do is provide a callback that can compute
the Color for each pixel based on the original Color.
