From ngld and Parias: Fix for Mantis 3084: Bitmaps loaded with gr.loadTexture() are never released.

git-svn-id: svn://svn.icculus.org/fs2open@10970 387891d4-d844-0410-90c0-e4c51a9137d3
From karajorma: Adds the multi-eval SEXP
WARNING: This SEXP is subject to change and will likely be removed or changed in a future version, use with caution.
Also fixes Mantis 3079: Add multi-eval to list of available SEXPs (.patch included)

git-svn-id: svn://svn.icculus.org/fs2open@10971 387891d4-d844-0410-90c0-e4c51a9137d3