Blender Git Loki

Cycles: Write samples count pass

Done from the path initialization function as it is a single kernel in
which it is known path will be terminated (if path was initialized, it
will be followed along and eventually terminated). So instead of doing
the write in every kernel where path terminates do it in the single
place.

Needed to pass render buffers along, but this is more of a technical
change.

The sample number is always positive, as we no longer have multiple
tiles rendering at the same time. Probably logic somewhere else can
be simplified now.

The pass is only available if the rendering did finish. This is due to
the way how PathTrace communicates passes to Blender side.