Ensure that the linear fit passes through the last (D) point

With a higher tolerance, the previous algorithm allowed the line to
drift up as the curve became exponential. This led to larger
discontinuities at D. The new algorithm keeps the same error guarantee,
but also ensures that the line passes exactly through the D point.

Bug: skia:
Change-Id: Ifaa6e57da55783ed1452a2cc0f38591fa4fb7dbb
Reviewed-on: https://skia-review.googlesource.com/117632
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>
15 files changed