Improve Cycles displacement

Cycles currently supports displacement in a very limited form. Bump mapping only in normal mode, actual mesh displacement and auto-bump are available in experimental mode. There are some issues updating displacement with the viewport renderer, and there is not currently a viable way to tesselate meshes at render time. The existing dicing system in experimental mode destroys face data - including UVs! - making it essentially useless. A proper render-time subdivision system would be needed.In addition, only scalar displacement maps are supported. Vector/tangent vector support would be nice, preferably while retaining "both" mode/auto-bump support to avoid needing an additional normal map for high-res sculpts that won't be tesselated back to their highest level.