Just like svg, Lottie files can contain raster images, either embedded/encoded in the Lottie file itself, or as URL reference to external files. They can only appear in top-level assets, which can only be referenced from Image layers. Adding support includes

1. Adding the various asset and path handling options from svgtoqml to lottietoqml
2. Fix up the QLottieImage class. Image handling is implemented in this virtual Lottie item, which does not correspond to an actual Lottie element, but to the static top-level asset that specifies the image. All fine, but the class still has implemented "position", "radius" and "center" properties, that do not make sense at all; there are no such info in the Lottie asset. Instead there needs to be (static) width, height, and url properties, which are.
3. Implement the QLottieVisitor::render(QLottieImage&) method, using the info from the points above to fill in an ImageNodeInfo structure and call generateImageNode()
4. Add a Lancelot test with embedded image, and possibly other tests to check the asset handling options (check how is this for svgtoqml)