[skottie] Add line-spacing animator support

AE allows animating the line spacing text property [1].

Observed semantics:

  - spacing is applied as an offset to all fragments in a line
  - for selector/partial coverage, the spacing for a given line
    is the average of the computed spacing for each fragment
  - spacing is cumulative (applies to all lines following)

Plumb the new animator prop ("ls") and expand the existing line
tracking logic to also apply computed spacing offsets.

(also requires a Bodymovin update to export the line spacing property)

[1] https://helpx.adobe.com/after-effects/using/animating-text.html#text_animator_properties

Change-Id: I5517acea8dbc1b2fbae09cb0874f1e53cd2acb90
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/300377
Commit-Queue: Florin Malita <fmalita@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
6 files changed