Added a number of new cog verbs and bugfixes.

Added gl_FragDepth to ceiling and horizon sky shaders so that models can appear to pass through them. Modified script presenter to include dummy script instances, preserving level cog indices, and updated presenter and level loader to correctly populate level cog symbols. Added sighted flag to thingflags and implemented sighted message. Updated virtualfilesystem to automatically lowercase asset filenames, to simplify resource overrides on case-sensitive filesystems. Added initial work on keyframe mixing. Added various sound and message verbs.