GrTessellator: fix for vertex coincident with enclosing edge.

GrTessellator: fix for vertex coincident with enclosing edge.

If a previously-enclosing edge coincides exactly with the current
vertex, there are no two adjacent edges which enclose the vertex.
Since find_enclosing_edges() ensures that the left enclosing edge
is to the left of the vertex, the fix is to split the right
enclosing edge on the current vertex and restart intersection
tests.

Bug: 716720
Change-Id: Id26c5b92a6d6139f348e99554638cded37e81a8e
Reviewed-on: https://skia-review.googlesource.com/15261
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Stephen White <senorblanco@chromium.org>