Number of threads to use for rendering is now auto-detected by default.
Modified ChunkedArray class to conform better to stl containter interfaces.
Fixed bug in microsurface code more properly.

The microsurface node struct has a field "child_index" that wasn't large
enough to address all of the nodes when there were more than one time sample.

For now, I'm just making that field larger.  In the future this can be
dealt with better by using e.g. an implicit indexing scheme.