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, 18 insertions, 0 deletions

diff --git a/NEWS b/NEWS
index 9c798dd2..a4a2c83d 100644
--- a/NEWS
+++ b/NEWS
@@ -69,6 +69,24 @@ New experimental backends:
 	    more offloading onto the GPU.
 	    The initial work on the backend was performed by Eric Anholt.
 
+Long standing bugs fixed:
+
+  Self-intersecting strokes.
+
+    A long standing bug where the coverage from overlapping semi-opaque
+    strokes (including neighbouring edges) was simply summed in lieu of
+    a costly global calculation has been fixed (by performing the costly
+    global calculation!) In order to mitigate the extra cost, the
+    tessellator has been overhauled and tune, which handles the fallback
+    for when we are unable to use the new span rasteriser on the stroke
+    (e.g. when using the current RENDER protocol). The large number of
+    pixel artefacts that implementing self-intersection elimination
+    removes is ample justification for the potential performance
+    regression. If you unfortunately do suffer a substantial performance
+    regression in your application, please consider obtaining a
+    cairo-trace and submitting it to us for analysis and inclusion into
+    our performance suite.
+
 
 Snapshot 1.9.2 (2009-06-12)
 ===========================