Cycles: add rejection of inf/nan samples, in principle these should not happen

but this makes it more reliable for now.

Also add an integrator "Clamp" option, to clamp very light samples to a maximum
value. This will reduce accuracy but may help reducing noise and speed up
convergence.

intern/cycles/render/integrator.h

@@ -45,6 +45,8 @@ public:
 	int seed;
 	int layer_flag;
 
+	float sample_clamp;
+
 	bool need_update;
 
 	Integrator();