Ensure that the TFs from ApproximateCurve are invertible

It's possible to produce an inverse curve (tf_inv) whose inverse (tf) is
no longer invertible.

Bug: chromium:841210
Change-Id: I757330039191da2aa74ad8a091d679f987c940bf
Reviewed-on: https://skia-review.googlesource.com/126924
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>
14 files changed