Check render buffer type, when binding it in a display list.

Previously, the display list just assumed that you knew what you were
doing.  But since I have some data available, might as well throw an
error if you're doing something silly.

I'm totally not making this change because I spent the two hours
debugging why my geometry wasn't rendering correctly, when I was
stupidly calling "VertexBuffer" when I should have been calling
"BindBuffer".

One of these days I should really upgrade VectorStorm to use OpenGL 3;
"custom" buffer types don't really make sense at all until then.