If you want to use any images in your geojson that are under your
drawables dir on Android and *.xcassets on iOS you now can provide
an assets key to the images prop for the ShapeSource and the native code
will add these images to the map style for you.
There seem to be some styles that will be special cases to the rules used from the json spec
this PR adds the ability to add overrides, so we can be flexible in special cases. In this case
we were using a NSArray for textOffset and iconOffset when it should be a CGVector.
Nick