* 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/iglrender.h

@@ -82,6 +82,7 @@ public:
 	GLint m_texture5;
 	GLint m_texture6;
 	GLint m_texture7;
+	GLint m_textureSkyBox;
 	Vector4 m_colour;
 	GLenum m_blend_src, m_blend_dst;
 	GLenum m_depthfunc;

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;

include/itextures.h

@@ -32,8 +32,9 @@ class LoadImageCallback
 public:
 	void* m_environment;
 	LoadFunc m_func;
+	bool m_skybox;
 
-	LoadImageCallback( void* environment, LoadFunc func ) : m_environment( environment ), m_func( func ){
+	LoadImageCallback( void* environment, LoadFunc func, bool skybox = false ) : m_environment( environment ), m_func( func ), m_skybox( skybox ){
 	}
 	Image* loadImage( const char* name ) const {
 		return m_func( m_environment, name );