More bidir optimisations

A little while ago (1st december last year) I wrote that I had achieved a 20% speedup of bidir (bidirectional path tracing) on a reference test scene, measured relative to our last stable release, 3.6.

Since then I've been continuing to optimise bidir and the indigo render core in general. Indigo now renders 1.8x faster on the test scene! (80% faster).

This is measured on a pretty simple scene - speedups for more complex scenes will be somewhat less. However I think it's a pretty big speed boost for what will be a free upgrade (version 3.8) for all Indigo 3 owners.

-- nick