Split VelloAsset into VelloSvg and VelloLottie.

[simbleau]

Render logic is shared for both file types, and has shown to be a pretty clear bad pattern after some time. This means every SVG pays a "lottie" tax, even though it's static. There's also logic and components that only work for Lotties, but can be added to SVGs, such as the Theme component which does color remapping (only works for lottie).

The issue is simple, split VelloAsset in two.