This is mostly a straightforward conversion, with a couple of
interesting notes:

* The previous radial gradient shader was missing a transform
  implementation so would never have worked properly with
  rotation and/or perspective.
* We didn't (and still don't) detect if a radial gradient is
  opaque - it's always assumed transparent. So even though
  it can now opt in to segments (and save clip mask generation
  time), the inner segments will still be in the alpha pass.
  We should fix this!
* Previously, we used to append the gradient stops directly
  after the primitive information in the main GPU cache handle.
  Now, there is a separate GPU cache handle for the stops. This
  is quite a bit more efficient in the GPU cache, since the
  size of the stops array is large, and a power of 2, it now
  fits exactly into a GPU cache slab, and allows the main
  primitive data to be allocated in a smaller row.