optimize pathops coverage

Remove unused code from SkOpSegment.cpp and friends.
Add new tests exposed by coverage.
Fix a bug exposed by coverage -- removing the need to detect points that are nearby when intersecting.
Add gyp rule for building coverage flavor on Mac.

R=mtklein@google.com

Author: caryclark@google.com

Review URL: https://codereview.chromium.org/75453003

git-svn-id: http://skia.googlecode.com/svn/trunk@12344 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/pathops/SkOpAngle.cpp b/src/pathops/SkOpAngle.cpp
index 4144add..83ca55c 100644
--- a/src/pathops/SkOpAngle.cpp
+++ b/src/pathops/SkOpAngle.cpp
@@ -108,7 +108,9 @@
     }
     // see if either curve can be lengthened before trying the tangent
     if (fSegment->other(fEnd) != rh.fSegment  // tangents not absolutely identical
-            && rh.fSegment->other(rh.fEnd) != fSegment) {  // and not intersecting
+            && rh.fSegment->other(rh.fEnd) != fSegment
+            && y != -DBL_EPSILON 
+            && ry != -DBL_EPSILON) {  // and not intersecting
         SkOpAngle longer = *this;
         SkOpAngle rhLonger = rh;
         if ((longer.lengthen(rh) | rhLonger.lengthen(*this))  // lengthen both