[bdpt] C++ versions differs from Rust version

[wahn]

v0.7.3 still renders the same as the C++ version. Somewhere after that a bug was introduced. Needs debugging.

[wahn]

$ git describe --tags
v0.7.3
$ imf_diff pbrt.png pbrt_cpp.png 
pbrt.png pbrt_cpp.png: no differences.
== "pbrt.png" and "pbrt_cpp.png" are identical

[wahn]

$ git checkout v0.8.0
$ make
$ ./target/release/rs_pbrt speed_test.pbrt
$ imf_diff pbrt.png pbrt_cpp.png
differing pixels:	 80.053% (50033 of 62500)
average difference:	  4.646%
maximum difference:	 14.728%
Summary: Many pixels differ.
== "pbrt.png" and "pbrt_cpp.png" are different

[wahn]

Commit b7a3e3d seems to introduce the bug ...

[wahn]

$ git describe --tags
v0.8.2-62-gf2cd4ae
$ imf_diff pbrt_cpp.png pbrt.png 
pbrt_cpp.png pbrt.png: no differences.
== "pbrt_cpp.png" and "pbrt.png" are identical