Cycles: Adding field-of-view options to the equirectangular panorama camera

This patch adds the option to set minimum/maximum latitude/longitude values for
the equirectangular panorama camera in Cycles, as discussed in T34400.

The separate functions in kernel_projection.h are needed because the regular
ones are also used as helper functions for environment map sampling.

Reviewers: #cycles, sergey

Reviewed By: #cycles, sergey

Subscribers: dingto, sergey, brecht

Differential Revision: https://developer.blender.org/D960

intern/cycles/render/camera.h

@@ -53,6 +53,10 @@ public:
 	PanoramaType panorama_type;
 	float fisheye_fov;
 	float fisheye_lens;
+	float latitude_min;
+	float latitude_max;
+	float longitude_min;
+	float longitude_max;
 
 	/* anamorphic lens bokeh */
 	float aperture_ratio;