World textures displaying for viewport in BI.

This patch supports "Image or Movie" and "Environment map" types of world texture for the viewport.
It supports:

  - "View", "AngMap" and "Equirectangular" types of mapping.

  - Different types of texture blending (according to BI world render).

  - Same color blending as when it lacked textures (but render via glsl).

{F207734}
{F207735}

Example: {F275180}
Original author: @valentin_b4w

Regards,
Alexander (Blend4Web Team).

Reviewers: sergey, valentin_b4w, brecht, merwin

Reviewed By: merwin

Subscribers: campbellbarton, merwin, blueprintrandom, youle, a.romanov, yurikovelenov, AlexKowel, Evgeny_Rodygin

Projects: #rendering, #opengl_gfx, #bf_blender:_next

Differential Revision: https://developer.blender.org/D1414

source/blender/gpu/intern/gpu_codegen.h

@@ -63,7 +63,8 @@ typedef enum GPUDataSource {
 typedef enum {
 	GPU_NODE_LINK_IMAGE_NONE = 0,
 	GPU_NODE_LINK_IMAGE_BLENDER = 1,
-	GPU_NODE_LINK_IMAGE_PREVIEW = 2
+	GPU_NODE_LINK_IMAGE_PREVIEW = 2,
+	GPU_NODE_LINK_IMAGE_CUBE_MAP = 3
 } GPUNodeLinkImage;
 
 struct GPUNode {