This patch switches from boost's filesystem v2 to v3.
This should be completely smooth due to filesystem v3 is pretty
old already.
Patch by Sven-Hendrik Haase (aka svenstaro), thanks!
disk to be reused by the next render.
This is useful for rendering animations where only the camera or materials change.
Note that saving the BVH to disk only to be removed for the next frame is slower
if this is not the case and the meshes do actually change.
For a render, it will save bvh files to the cache user directory, and remove all
cache files from other renders. The files are named using a MD5 hash based on the
mesh, to verify if the meshes are still the same.
