This is a big step towards a cleaner api. The struct reference in
model_t really should be a pointer, but bsp submodel(?) loading messed
that up, though that's just a matter of taking more care in the loading
code. It seems sensible to make that a separate step.
Where possible, symbols have been made static, prefixed with gl_/GL_ or
moved into the code shared by all renderers. This will make doing plugins
easier but done now for link testing.
