Cycles: Add option to replace GI with AO approximation after certain amount of bounces

This is a speed up option which is mainly useful for viewport. Gives nice speedup in
the barbershop scene of 2x when replacing GI with AO after 2nd bounce without loosing
too much details.

Reviewers: brecht

Subscribers: eyecandy, venomgfx

Differential Revision: https://developer.blender.org/D2383

intern/cycles/render/integrator.h

@@ -43,6 +43,8 @@ public:
 	int transparent_max_bounce;
 	bool transparent_shadows;
 
+	int ao_bounces;
+
 	int volume_max_steps;
 	float volume_step_size;