Split up refEntity_t?

[zturtleman]

Most of refEntity_t is only used by models. Currently sprite radius/rotation is in it, but only used in cgame.

Maybe let cgame have it's own generic refresh entity and have Renderer API can be more specific, though initially it will just be converted into a refent in renderer.

There is quite a bit of duplicated vars needed for shader system (for both model and poly refents), making storing the data in renderer a bit more complicated?

model entity struct and syscall
poly entity struct and syscall

syscall for adding portal sufaces (doesn't need to be a refent, API-wise or usage in cgame)