[skottie] Refactor/generalize text selector shape generators

The main motivation for this change is getting *all* shape generators to
run the ramp through a cubic mapper - this will allow adding
ease-in/ease-out support in a follow up CL.

As a bonus, the new implementation is more straightforward and concise
(at the expense of dropping some likely-premature optimizations).

Change-Id: Ia2bc53525ae529c35300be124220b4d61e4fc474
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/238061
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
1 file changed