* Allow external images to be loaded by LottieVisualSource, and add .lottie file support.
External images (i.e. images that are not embedded in the .JSON file) are a problem for LottieVisualSource because it doesn't know how to get them. For securit reasons, we don't want LottieVisualSource to have the ability to read files that are referenced from a Lottie .json file, and in many cases LottieVisualSource wouldn't be able to resolve the file because it doesn't have permissions.
This change adds an ImageAssetDelegate that the user can set on the AnimatedVisualSource in order to handle reading of external images.
This change also adds support for the .lottie format (see https://dotlottie.io). There is a parser for .lottie files, and support in LottieViewer for displaying Lotties from a .lottie file. Because .lottie files package images together with the .json Lottie file, it enables LottieViewer to display Lotties that have external images.
* Fix some bugs that were obvious when I started to CR.
* CR feedback.
* CR feedback.
* CR feedback.
* Fix typo in header.
aborziak-ms
Simeon