Add copy constructor and op= to BDPT Vertex class
Add copy constructor and op= to BDPT Vertex class

This seems to be what we need to make compilers happy with non-POD types in the anonymous union. (Tested on a range of compilers that previously failed with this.)

Issues #8, #52, #5.

nispaur/pbrt-v3,bssrdf/pbrt-v3,ambakshi/pbrt-v3,nispaur/pbrt-v3,nispaur/pbrt-v3,nyue/pbrt-v3,mmp/pbrt-v3,nispaur/pbrt-v3,ambakshi/pbrt-v3,mmp/pbrt-v3,bssrdf/pbrt-v3,nyue/pbrt-v3,ambakshi/pbrt-v3,shihchinw/pbrt-v3,mmp/pbrt-v3,bssrdf/pbrt-v3,nyue/pbrt-v3,shihchinw/pbrt-v3,mwkm/pbrt-v3,nyue/pbrt-v3,shihchinw/pbrt-v3,shihchinw/pbrt-v3,mmp/pbrt-v3,mwkm/pbrt-v3