Cycles: fix bugs in point and spot light multiple importance sampling

* Spot lights are now handled as disks aligned with the direction of the
spotlight instead of view aligned disks.

* Point light is now handled separately from the spot light, to fix a case
where multiple lights are intersected in a row. Before the origin of the
ray was the previously intersected light and not the origin of the initial
ray traced from the last surface/volume interaction.

This makes both strategies in multiple importance sampling converge to the same
result. It changes the render results in some scenes, for example the junkshop
scene where there are large point lights overlapping scene geometry and each
other.

Differential Revision: https://developer.blender.org/D13233

Cycles: fix bugs in point and spot light multiple importance sampling

* Spot lights are now handled as disks aligned with the direction of the
spotlight instead of view aligned disks.

* Point light is now handled separately from the spot light, to fix a case
where multiple lights are intersected in a row. Before the origin of the
ray was the previously intersected light and not the origin of the initial
ray traced from the last surface/volume interaction.

This makes both strategies in multiple importance sampling converge to the same
result. It changes the render results in some scenes, for example the junkshop
scene where there are large point lights overlapping scene geometry and each
other.

Differential Revision: https://developer.blender.org/D13233

Cycles: add build option to enable a debugging feature for MIS

This patch adds a CMake option "WITH_CYCLES_DEBUG" which builds cycles with
a feature that allows debugging/selecting the direct-light sampling strategy.
The same option may later be used to add other debugging features that could
affect performance in release builds.

The three options are:
* Forward path tracing (e.g., via BSDF or phase function)
* Next-event estimation
* Multiple importance sampling combination of the previous two methods

Such a feature is useful for debugging light different sampling, evaluation,
and pdf methods (e.g., for light sources and BSDFs).

Differential Revision: https://developer.blender.org/D13152