Spotlights: Spotlights do no reflect off mirrors

[gkjohnson]

Related to #244

Spotlights are currently disabled for non-MIS rendering because with the current implementation the visual result does not match the MIS code path. This means that the spot lights do not bounce from reflections. See the following resources on how to adjust the PDF and implement punctual light sampling:

• https://www.pbr-book.org/3ed-2018/Light_Sources/Point_Lights#Spotlights
• https://www.pbr-book.org/3ed-2018/Light_Transport_I_Surface_Reflection/Sampling_Light_Sources#LightswithSingularities

[gkjohnson]

Its possible that more bidirectional path tracing capabilities are needed: https://www.pbr-book.org/3ed-2018/Light_Transport_III_Bidirectional_Methods/Bidirectional_Path_Tracing