Fix track image bleeding through ship engine area

[jnmartin84]

With a small modification to objects_load (a two-pass load that skips PRM_SHIP_ENGINE primitives during the first pass), all of the exhaust primitives can be appended to the end of the primitive list for ships.

This fixes the track image bleed and does not require messing with depth write / depth test.

Sorry I am bad at git, here's my second try with a clean commit history.

[jnmartin84]