[skottie] Initial text range selector support

Each animator can have multiple range selectors, whose combined "coverage"
modulates how the animator props compose with other/initial props.

Since there can be multiple animators with different/arbitrary selectors,
we compute independent property values for each fragment.

Supported features:

 - start, end, offset, amount
 - units: percentage, index
 - based-on: characters-only for now
 - mode: add-only for now
 - shape: square-only for now

Change-Id: If7fee46ffb29e1f92542822481ed699fd0b0b521
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/218076
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Ben Wagner <bungeman@google.com>
1 file changed