Since SVG gradients can reference each other, we have to keep track of
visited gradients when traversing the link chain, or we will recurse
infinitely when there's a reference cycle.
This commit removes DeprecatedString's "null" state, and replaces all
its users with one of the following:
- A normal, empty DeprecatedString
- Optional<DeprecatedString>
Note that null states of DeprecatedFlyString/StringView/etc are *not*
affected by this commit. However, DeprecatedString::empty() is now
considered equal to a null StringView.
This is the base class for all SVG gradient types. This supports:
- The `gradientUnits` attribute
- The `gradientTransform` attribute
- And following `xlink:hrefs` for inheriting <stops>/attributes
