Handle degenerate curve segments for marker orientation

When a cubic or quadratic curve have a point that coincide with another
of its point such that either the start or end tangent vector is
degenerate, the orientation in that point will be 0. In such cases try
to use the next/previous non-degenerate tangent vector (if any) instead.

Bug: 450368
Change-Id: I35db404b3f82cf61b10427678feecc2157eeeb26
Reviewed-on: https://chromium-review.googlesource.com/1196503
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#587743}
5 files changed