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.
The translation to the bounding box location is handled by the gradient
transform, also doing it here breaks things.
This fixes the MDN <radialGradient> example.
This follows on from the SVG linear gradients. It supports the same
features (xlink:href, gradientUnits, gradientTransform).
With this commit I have now implemented all web gradients :^)
(Though we are still missing a few parameters for SVG gradients,
e.g. spreadMethod).
