We were missing the use case when a user wants to use a custom sprite
this now checks to make sure that we only add an image to the map style
if it has a valid uri/url
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