Gradients referencing other gradients that are defined later

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.

Start by tracing how xlink:href resolves gradient references during SVG Native document traversal, then reproduce the case with a gradient that references one defined later, as described in the issue. The fix is complete when later-defined gradients resolve correctly and traversal reports an error if references remain unresolved at the end.

Written by the indexing model from the issue text.