lottie: support video playback in image assets

Reuse the existing Lottie image asset model and automatically play media
when an image asset contains video data such as MP4.

Keep the asset data URI under the "data:image/mp4" format instead of
introducing a video-specific asset type, preserving compatibility with
the existing Lottie specification.

Drive playback from layer progress and stop media when the image layer
leaves its active frame range.

issue: https://github.com/thorvg/thorvg/issues/4635