Fix GrAAConvexTessellator colinear point removal.

We weren't checking whether the new point in lineTo was backtracking or not.

Also:
Replace distance-to-line check with triangle area check. The previous check
was asymmetric. Given point sequence (a, b, c) it might make a different
decision than when given (c, b, a).

Compute normals late since we don't use them to detect colinear edges
anymore.

Rename SkPointPriv::SetOrhog -> SkPointPriv::MakeOrthog and return
computed value rather than take SkPoint* dst.

Bug: chromium:869172


Change-Id: I8da53edf1a2e6098f4199da57368ebb644866e4c
Reviewed-on: https://skia-review.googlesource.com/150682
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
7 files changed