Matcap support in 3D Viewport.

Full log is here:
http://wiki.blender.org/index.php/Dev:Ref/Release_Notes/2.66/Usability#Matcap_in_3D_viewport

Implementation notes:

- Matcaps are an extension of Solid draw mode, and don't show in other drawmodes.
  (It's mostly intended to aid modeling/sculpt)

- By design, Matcaps are a UI feature, and only stored locally for the UI itself, and
  won't affect rendering or materials.

- Currently a set of 16 (GPL licensed) Matcaps have been compiled into Blender. 
  It doesn't take memory or cpu time, until you use it.

- Brush Icons and Matcaps use same code now, and only get generated/allocated on
  actually using it (instead of on startup).

- The current set might get new or different images still, based on user feedback.

- Matcap images are 512x512 pixels, so each image takes 1 Mb memory. Unused matcaps get 
  freed immediately. The Matcap icon previews (128x128 pixels) stay in memory.

- Loading own matcap image files will be added later. That needs design and code work 
  to get it stable and memory-friendly.

- The GLSL code uses the ID PreviewImage for matcaps. I tested it using the existing
  Material previews, which has its limits... especially for textured previews the
  normal-mapped matcap won't look good.

source/blender/gpu/intern/gpu_codegen.h

@@ -43,6 +43,7 @@ struct GPUOutput;
 struct GPUNode;
 struct GPUVertexAttribs;
 struct GPUFrameBuffer;
+struct PreviewImage;
 
 #define MAX_FUNCTION_NAME	64
 #define MAX_PARAMETER		32
@@ -138,6 +139,7 @@ typedef struct GPUInput {
 
 	struct Image *ima;		/* image */
 	struct ImageUser *iuser;/* image user */
+	struct PreviewImage *prv;	/* preview images & icons */
 	int image_isdata;		/* image does not contain color data */
 	float *dynamicvec;		/* vector data in case it is dynamic */
 	int dynamictype;		/* origin of the dynamic uniform (GPUDynamicType) */