Added "refract" pass to default renderpaths. This has the rendered scene so far (opaque + sky) available in the environment texture unit.
Renamed the "prealpha" pass to "postopaque" (will break custom techniques and renderpaths referring to prealpha.)
If a scene pass has nothing to render, skip it completely during View processing, including rendertarget allocations. This makes the refract pass free when there is no refractive geometry.
Changed AssetImporter to save the relevant scene node hierarchy in scene mode, instead of just nodes containing meshes.
Added node (prefab) save mode to AssetImporter.
Improved "Scene model" documentation page to mention scene load/save and prefab instantiation.