fix bugs found by computing flat clips in 800K skps

Enabling the canvas bit to turn the clip stack into a flat replace exposed around 100 failures when testing the 800K skp set generated from the top 1M web sites.

This fixes all but one of those failures.

Major changes include:
- Replace angle indices with angle pointers. This was motivated by the need to add angles later but not renumber existing angles.
- Aggressive segment chase. When the winding is known on a segment, more aggressively passing that winding to adjacent segments allows fragmented data sets to succeed.
- Line segments with ends nearly the same are treated as coincident first.
- Transfer partial coincidence by observing that if segment A is partially coincident to B and C then B and C may be partially coincident.

TBR=reed

Committed: https://skia.googlesource.com/skia/+/dac1d17027dcaa5596885a9f333979418b35001c

[caryclark, 2014-06-16 20:06:22]

The CQ bit was checked by caryclark@google.com

[commit-bot: I haz the power, 2014-06-16 20:07:11]

CQ is trying da patch. Follow status at
 https://skia-tree-status.appspot.com/cq/caryclark@google.com/272153002/300001

[caryclark, 2014-06-17 11:59:30]

The CQ bit was checked by caryclark@google.com

[commit-bot: I haz the power, 2014-06-17 12:00:04]

CQ is trying da patch. Follow status at
 https://skia-tree-status.appspot.com/cq/caryclark@google.com/272153002/320001

[commit-bot: I haz the power, 2014-06-17 12:15:48]

Change committed as dac1d17027dcaa5596885a9f333979418b35001c