Trunk sync r10479.

git-svn-id: svn://svn.icculus.org/fs2open@10480 387891d4-d844-0410-90c0-e4c51a9137d3
going through old revisions, r8540 is wrong, since it was like that in trunk -- memsetting the whole struct to -1 sets all integers and floats to -1
(so let's go back to the old way in case some old retail code relied on model_instance_num, model_num, or submodel_num being set to -1)

git-svn-id: svn://svn.icculus.org/fs2open@10481 387891d4-d844-0410-90c0-e4c51a9137d3