Cycles: Add an interpolation option to environment textures

This commit exposes the interpolation parameter for environment textures (requested by DolpheenDream on IRC), just as it already is for image textures.

Reviewers: sergey

Differential Revision: https://developer.blender.org/D1544

intern/cycles/render/nodes.h

@@ -116,6 +116,7 @@ public:
 	void *builtin_data;
 	ustring color_space;
 	ustring projection;
+	InterpolationType interpolation;
 	bool animated;
 
 	static ShaderEnum color_space_enum;