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.

Makefile

@@ -19,11 +19,15 @@ bin/raytracer: $(OBJ_FILES)
 	@mkdir -p $(OBJ_DIR)
 	$(CXX) $(CXXFLAGS) -o $@ $^
 
+.PHONY: randsampl
+randsampl: CXXFLAGS += -DRANDOM_SAMPLING
+randsampl: bin/raytracer
+
 .PHONY: debug
 debug: CXXFLAGS += -O0 -g
 debug: clean bin/raytracer
 
 .PHONY: clean
 clean:
-	$(RM) -r obj/*.o
-	$(RM) bin/*
+	$(RM) obj/*.o
+	$(RM) bin/raytracer