BGE: Fix for bug #34377 "Multi UV mesh's materials not backwards compatible and Odd UV Coord behavior" reported by Alex Mairs (ctbm). The problem was the unused texture slot was still trying to be used. Now unused texture slots' mappings are set to DISABLE so they don't mess up the rest of the conversion process.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54780 954f8c5b-7b00-dc11-b283-0030488c597c