Implement a VideoAsset wrapper, used for Skottie video layers. This
requires a non-testlib build target for SkVideoDecoder, hence a
dedicated BUILD.gn.
Add software conversion fallback for SkVideoDecoder, using libswscale.
Change-Id: I80dd555a1241081e50ee4834b64ad3518948a0f1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/285378
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
