Implement depth of field

[wahn]

Chapter 6.2.3 of the PBRT book deals with The Thin Lens Model and Depth of Field. An example scene showing the effect:

Projective cameras take two extra parameters for depth of field: one sets the size of the lens aperture, and the other sets the focal distance:

> rg -A 1 -B 2 lensradius assets/scenes/yeahright/
assets/scenes/yeahright/yeahright.pbrt
9-Camera "perspective" 
10-        "float fov" [ 35 ] 
11:        "float lensradius" [ 0.5 ] 
12-        "float focaldistance" [ 50 ] 

[wahn]

Commit 276dc06 solves the issue:

There's still a difference to the C++ version though:

> imf_diff -d -f yeahright.exr pbrt_rust.exr diff.jpg
differing pixels:	 51.722% (331021 of 640000)
average difference:	  1.326%
maximum difference:	  5.591%
Summary: Many pixels differ slightly.
== "yeahright.exr" and "pbrt_rust.exr" are different

But you can clearly see the depth of field effect.

[wahn]

Closing the issue.