Currently, the code just always uses the regular MIS model with one light and one BSDF sample that's been used for path tracing.
For BPT, however, that is not really great since the user can control the amount of samples taken there. Therefore, while it still converges to the correct result, there's unneccessary noise when using very different values for light and BSDF samples.
This patch makes the MIS calculation account for sample numbers when using BPT, which can lead to a huge decrease in noise in some scenes (examples following soon). PT results are unchanged.