Improve tessellating path rasterizer performance on dashed paths.

Improve tessellating path rasterizer performance on dashed paths.

With dashed paths, we get a lot of geometry on the same sort line.
Since the vertices are sorted, it makes more sense to insert edges
from the end (in sort order) than the beginning. This requires
maintaining both the head and tail of the active edge list.

This gives a ~20% perf improvement on tabl_digg.skp.

BUG=skia:3733
Committed: https://skia.googlesource.com/skia/+/5b9f42c4d1ddc776334b3de676450f4cdaf607bd

[Stephen White, 2015-04-16 14:10:01]

senorblanco@chromium.org changed reviewers:
+ egdaniel@google.com

[Stephen White, 2015-04-16 14:10:02]

Greg: PTAL. Thanks!

[egdaniel, 2015-04-16 18:27:10]

lgtm. One general question, so dashing speeds up when you start at the end (as
per your numbers), was there anything that actually slowed down? Or is it the
case the everything is always the same/faster from starting at the end?

[Stephen White, 2015-04-16 18:34:52]

The CQ bit was checked by senorblanco@chromium.org

[commit-bot: I haz the power, 2015-04-16 18:35:13]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1089903002/20001

[Stephen White, 2015-04-16 18:38:42]

On 2015/04/16 18:27:10, egdaniel wrote:
> lgtm. One general question, so dashing speeds up when you start at the end (as
> per your numbers), was there anything that actually slowed down? Or is it the
> case the everything is always the same/faster from starting at the end?

None of the test cases I tried regressed. The only case that I can conjure which

would slow down is one where an object is "dashed" along the top edge, but
rotated
slightly counter-clockwise, so that the y values decrease but the dashes still
overlap in Y.

I'm not too worried about that case, though, since we always tessellate in 
local space, so it won't happen for rotated dashed rects, and we're about 
to introduce other heuristics to ameliorate the dashing problem more generally.

[commit-bot: I haz the power, 2015-04-16 18:47:21]

Committed patchset #2 (id:20001) as
https://skia.googlesource.com/skia/+/5b9f42c4d1ddc776334b3de676450f4cdaf607bd