Cycles: First implementation of shadow catcher

It uses an idea of accumulating all possible light reachable across the
light path (without taking shadow blocked into account) and accumulating
total shaded light across the path. Dividing second figure by first one
seems to be giving good estimate of the shadow.

In fact, to my knowledge, it's something really similar to what is
happening in the denoising branch, so we are aligned here which is good.

The workflow is following:

- Create an object which matches real-life object on which shadow is
  to be catched.

- Create approximate similar material on that object.

  This is needed to make indirect light properly affecting CG objects
  in the scene.

- Mark object as Shadow Catcher in the Object properties.

Ideally, after doing that it will be possible to render the image and
simply alpha-over it on top of real footage.

intern/cycles/blender/blender_object.cpp

@@ -343,6 +343,13 @@ Object *BlenderSync::sync_object(BL::Object& b_parent,
 		object_updated = true;
 	}
 
+	PointerRNA cobject = RNA_pointer_get(&b_ob.ptr, "cycles");
+	bool is_shadow_catcher = get_boolean(cobject, "is_shadow_catcher");
+	if(is_shadow_catcher != object->is_shadow_catcher) {
+		object->is_shadow_catcher = is_shadow_catcher;
+		object_updated = true;
+	}
+
 	/* object sync
 	 * transform comparison should not be needed, but duplis don't work perfect
 	 * in the depsgraph and may not signal changes, so this is a workaround */