feat: impl random sampling instead of point light algorithm

Random sampling is available when building with `-DRANDOM_SAMPLING', or with
`make randsampl'.

The sample counts are rather low, so the output is both slow and noisy, which is
why it's disabled by default.

README

@@ -9,6 +9,7 @@ The raytracer implements the following:
  - IOR-based refraction
  - Cook-Torrance BRDF (a few other models are left in the code)
  - Primitive object materials
+ - Random sampling (build with `make randsampl' to add fireflies :)
 
 You can move around with WASD and SPC/Z; zoom with I/O, change the exposure with
 +/-, and quit with Q.