add -f mp4 to skottie_tool

This runs way faster than skottie2movie, even with Mike's patch.
We could probably ditch skottie2movie with this CL and one adding --gpu?

Change-Id: I41376c2cd636eb75a3a6d2aaa75bef48211a1021
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/247377
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Klein <mtklein@google.com>
2 files changed