v0.4 : Several QOL features

[simbleau]

Changed

• SVG (.svg) support is now added through a cargo feature svg.
• Lottie (.json) support is now added through the cargo feature lottie.
• experimental dotLottie features (LottiePlayer, PlayerTransition, PlayerState) are now feature-gated through the cargo feature experimental-dotLottie. This is only partial support, and a work in progress.
• VelloAsset.metadata() is no longer available, as it is specific to Lottie. There is now a trait, LottieExt that can be imported to call .metadata() on a Composition instead. This is no longer fallible as a result.
• PlaybackAlphaOverride was removed in favor of an alpha field on VelloAsset.
• LottiePlayer was renamed to DotLottiePlayer.

Fixed

• A slow startup delay for lottie assets to begin rendering
• A dotLottie issue where the first frame can jump on web platforms.