Eliminate self-intersecting strokes.

We refactor the surface fallbacks to convert full strokes and fills to the
intermediate polygon representation (as opposed to before where we
returned the trapezoidal representation). This allow greater flexibility
to choose how then to rasterize the polygon. Where possible we use the
local spans rasteriser for its increased performance, but still have the
option to use the tessellator instead (for example, with the current
Render protocol which does not yet have a polygon image).

In order to accommodate this, the spans interface is tweaked to accept
whole polygons instead of a path and the tessellator is tweaked for speed.

Performance Impact
==================

...
Still measuring, expecting some severe regressions.
...

1 files changed, 0 insertions, 0 deletions

diff --git a/test/clipped-trapezoids-ref.png b/test/clipped-trapezoids-ref.png
new file mode 100644
index 000000000..3fd300c04
--- /dev/null
+++ b/test/clipped-trapezoids-ref.png