* render Q3 shader based skyboxes

logic: load 6 skybox textures when shader gets used by scene, don't unload dynamically, just on 'flush'
texture browser only uses normal preview image and doesn't trigger potentially heavy box loading
also fix R_ResampleTexture for [2+x upscaling

include/ishaders.h

@@ -98,6 +98,7 @@ public:
 	virtual void DecRef() = 0;
 // get/set the qtexture_t* Radiant uses to represent this shader object
 	virtual qtexture_t* getTexture() const = 0;
+	virtual qtexture_t* getSkyBox() = 0;
 	virtual qtexture_t* getDiffuse() const = 0;
 	virtual qtexture_t* getBump() const = 0;
 	virtual qtexture_t* getSpecular() const = 0;