Gradients referencing other gradients that are defined later

[moazin]

At the moment, SNV does support one gradient using another one by the xlink:href attribute, however, it doesn't support it if the referenced element comes later on. This needs to be fixed as it's very common in fonts like Trajan. I'm thinking perhaps we could iterate and keep unresolved references and at the end of the traversal we can throw an error if unresolved references still exist.