[cairo] GSoC: Scan converting rasteriser update

[Jeff Muizelaar]

Some more review. I didn't get as much time to look at the new code today as I
would've liked, so this is all I have for now.

[cairo-spans] Implement a span renderer for cairo_image_surface_t.
- it might be good to separate out the _mask_init() and _mask_fini()
  changes into a separate patch. i.e. separate out the cosmetic
  changes from the behavioural changes.

- the function declaration style of _cairo_image_surface_span_renderer_render_row()
  and _cairo_image_surface_span_renderer_destroy() don't match the rest of the file.


-Jeff