shader editor: add skybox template

radiant/gtkdlgs.cpp

@@ -743,6 +743,22 @@ R"(
 	// remap back to original shader
 	q3map_remapShader textures/
 }
+)"
+	},
+	{
+		"%skybox",
+R"(
+	qer_editorImage env/
+	surfaceparm noimpact
+	surfaceparm nolightmap
+	surfaceparm sky
+	q3map_sunExt 1 1 1 85 -43 60 2 16
+	q3map_LightMapFilterRadius 0 8
+	q3map_skylight 70 4
+	nopicmip
+	// path without _bk.tga suffix
+	skyparms env/
+}
 )"
 	},
 };