20130121b7

- Internal tweaks to motion override code in scene generation, nothing
user-relevant.
- Camera override no longer has trouble with .mot files.
- Added error handling for invalid filenames in overrides. Avoids an
ugly error message from LScript that scares the user.
- Fixed p0031 - object override led to 0 KB scene file! Misfire in
MotionMixer function. Also affected surface overrides.