Cycles: Pole merging for spherical stereo

The idea of pole merge is to fade interocular distance after a certain
altitude to zero when altitude goes closer to a pole. This should prevent
annoyances looking up in the sky or down to the bottom.

Works for both panorama and perspective cameras when Spherical Stereo
is enabled.

Reviewers: dfelinto, brecht

Reviewed By: brecht

Subscribers: sebastian_k

Differential Revision: https://developer.blender.org/D1998

intern/cycles/render/camera.h

@@ -104,6 +104,9 @@ public:
 	bool use_spherical_stereo;
 	float interocular_distance;
 	float convergence_distance;
+	bool use_pole_merge;
+	float pole_merge_angle_from;
+	float pole_merge_angle_to;
 
 	/* anamorphic lens bokeh */
 	float aperture_ratio;