DoF: bug fixes and quality improvements
DoF: bug fixes and quality improvements


- Use the random offset to calculate the correct intersection radius.
  For the center sample, the ring radius is otherwise zero, which means
  any sample in the random disk with a CoC radius > 0 would intersect,
  and that seems wrong.

- The selected mip was off by one.
  We were always picking a mip too high because the CoC are in
  high resolution, but our mip-chain is 1/4 res from that.
  This improves rendering quality quite a lot.

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