modify function headers as suggested in PR #530

[Goober5000]

As @asarium commented in #530:

An Assertion that checks if the model instance actually belongs to the model number could catch some programming errors here.

On seconds thought, polymodel_instance contains the model number of the referenced polymodel so the model_num parameter isn't needed at all.

[Goober5000]

bump Can someone restart Travis?

[asarium]

I don't think that's possible when the build never started. Rebasing the branch should trigger a new build if there were changes in master.

[Goober5000]

In that case, a new commit should do it as well. I've got just the one in mind, but it will probably be tomorrow before it's ready to go in.

[Goober5000]

This new commit pulls forward some minor changes needed for PR #530 (which will again be rebased after this hits master) and extends those changes to other related functions. All the model_instance functions now work directly on the model instances, rather than detouring through the object.

This allows model_instances of all sorts (including skyboxes, asteroids, and weapons) to use the functions, rather than restricting usage by assuming an OBJ_SHIP. (Practically speaking, the vast majority of places where these functions are called are in the ship code anyway.)

[asarium]

Code looks good and a quick test run didn't show anything unusual.