compute split conic endpoints exactly

compute split conic endpoints exactly

The divide by w can generate slightly erroneous results even
for t == 0 or t == 1. The error in turn defeats detecting
a point in common for a pair of curves that travel in
opposite directions.

Instead, special case endpoints when the t is 0 or 1.

TBR=reed@google.com
BUG=514118
Committed: https://skia.googlesource.com/skia/+/bb134333648c5e44e00a95b8323033a6264ecae0

[caryclark, 2015-07-28 12:04:46]

The CQ bit was checked by caryclark@google.com

[commit-bot: I haz the power, 2015-07-28 12:04:57]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1259513004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1259513004/1

[commit-bot: I haz the power, 2015-07-28 12:12:25]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/bb134333648c5e44e00a95b8323033a6264ecae0