This lens shader be added DOF sample control, improve quality of DOF blur .

The cause of modify original lens shader is improve rendering speed .
Original lens shader(physical_lens_dof) only sample four times per pixel, but this number of sample is too low
for final production .
The only answer is increase Sample Level in Render Global Settings, but it is also the more expensive(inefficient,
because sample per pixel on global image) .
I add a control for number of DOF(Depth of field) sampling, then you can control DOF's sample times .
These sample only for DOF blur, so it is less expensive and more efficient .

Please visit Release notes for full description after unpack.

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