[skottie] More snug kTop Shaper alignment

The current implementation relies on SkShaper ascent values for top
text box alignment, but the results are not as visually accurate as AE
(or Lottie).

Use the computed tight bounds instead.

Change-Id: I4447a834fe3cae398fc887766daa68802e7f50a5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/206684
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
4 files changed