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).
