Cycles: Implement camera zoom motion blur

Works totally similar to camera motion blur and majority of the changes are
related on just passing extra arguments to sync() functions.

Couple of things still to look into:

- Motion pass will not include motion caused by the zoom.
- Only perspective cameras are supported currently.
- Motion is being interpolated on projected coordinates, which might give
  different results from constructing projection matrix from interpolated
  field of view.

  This could be good enough for us, but we need to consider improving this
  at some point.

Reviewers: juicyfruit, dingto

Reviewed By: dingto

Differential Revision: https://developer.blender.org/D1383

intern/cycles/render/camera.h

@@ -83,7 +83,9 @@ public:
 
 	/* motion */
 	MotionTransform motion;
-	bool use_motion;
+	bool use_motion, use_perspective_motion;
+	float fov_pre, fov_post;
+	PerspectiveMotionTransform perspective_motion;
 
 	/* computed camera parameters */
 	Transform screentoworld;