Improved SwiftUI Support

[benpious]

We currently have experimental support for SwiftUI. However, it has yet to be used in any real Apps, so there may be issues with the API that we need to address.

Our use of EnvironmentValues is also not currently ideal, as summarized here.

We may wish to reimplement VectorDrawableView using GeometryReader and Paths, which which could allow us to take advantage of the Environment colors provided by Apple if they improve the API to allow us to get a color for a String.