Add pointer functions to root of module.

This is similar to other modules that provide
simple wrappers.
Also fix texture coord pointer not having GL prefix
to function call.
Replace Buffer with TypedBuffer.

Buffer is removed and TypedBuffer is now Buffer.
TypedBuffer is more in keeping with PyGLy's design.
If you want Buffer, it's trivial to implement your own.
Buffer didn't really do anything.
Add some comments to Buffer class.
Add the ability to pass in an existing Buffer handle.