Support for (Steep) Parallax Mapping

Parallax mapping is an addition to normal mapping that is more realistic. It works by also adjusting the texture coordinates in a way that surfaces have more apparent depth and parallax effects.
Comparison can be found here:
http://graphics.cs.brown.edu/games/SteepParallax/compare.jpg

We could implement this by adding new texturestage modes, MParallax and eventually also MSteepParallax.
This would read the normal map from RGB channels of the texture and the displacement/height map from the Alpha channel. So, the user would be able to load their parallax/height map like this:
tex = loader.loadTexture("normalmap.png", "heightmap.png")
ts = TextureStage("parallax")
ts.setMode(TextureStage.MParallax)
model.setTexture(ts, tex)