* 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

setup/data/tools/gl/skybox_fp.glsl

@@ -0,0 +1,8 @@
+
+uniform samplerCube skybox;
+
+void main()
+{
+	//doing rotation/flip to fix skybox orientation
+	gl_FragColor = textureCube( skybox, vec3( -gl_TexCoord[0].y, gl_TexCoord[0].z, gl_TexCoord[0].x ) );
+}

setup/data/tools/gl/skybox_vp.glsl

@@ -0,0 +1,8 @@
+
+uniform vec3 u_view_origin;
+
+void main()
+{
+    gl_TexCoord[0] = vec4( ( gl_Vertex.xyz - u_view_origin ), 1 );
+    gl_Position = ftransform();
+}