When using MIS, the world is treated as regular light and in this case
we can now also limit the maximum amount of bounces, the background light
will contribute to the scene.
This can improve performance in some cases, where it's e.g. sufficient to
only have a contribution on first 1-2 bounces.
Examples can be found in the differential.
Differential revision: https://developer.blender.org/D1399
d54e77b66d