Some small fixes

I've checked in a few small bugfixes.

A rather dumb mistake had snuck into the standard vertex shader that caused the normal matrix to be applied twice and that had some interesting effects on the lighting.
I've also checked in a few tweaks on the height map shaders.
Finally there is a small fix in the getPixel method in my texture map library and I've added a function for calculating mipmaps. We're not using these yet but we will soon.

Sorry for the lack of updates but I've been very busy behind the scenes. I'm slowly building out the platformer but I want to be a little further along before I start writing up tutorials as I'm unsure about a few things at this point in time.
Also there are a few more enhancements I wish to make to the engine that are easier to demonstrate in my little 3D scene we've got right now.

Don't hold me to it but I'm planning the following sections in the near future:
- a way to switch between meshes with different LOD based on the distance of an object
- adding bounding boxes to limit how much we draw
- switching to a deferred renderer
- shadows

By then I think we'll be ready to jump back into the platformer.