[skottie] Use explicit font ascent in Shaper

Bodymovin exports an explicit ascent for each font, as a percentage of
the text size.

Change-Id: I25708944b2b79b42a6ccb05abbe002685e36dfa1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/223986
Reviewed-by: Ben Wagner <bungeman@google.com>
8 files changed