[skottie] skottie_tool updates
[skottie] skottie_tool updates

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 <a8262bf13ea8c26821e91ea9bca6740a77c96dcd@chromium.org>
Commit-Queue: Florin Malita <a8262bf13ea8c26821e91ea9bca6740a77c96dcd@google.com>

google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia