1) plumb a precomp interceptor to support nested animations, following
the same naming pattern as viewer and dm
2) clear background with white instead of transparent, to match other
tools
TBR=
Change-Id: Ic1d1f8c6493a3ca98a9b75f5e2aa2230a46f54d9
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/298139
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@google.com>
6499e7fb4c