Fix 1883030: Version 0.1 of the timeloop layer acted as if it wasn't there whenever...

Fix 1883030: Version 0.1 of the timeloop layer acted as if it wasn't there whenever its start_time wasn't strictly less than its end_time, whereas the current version works with both positive and negative values for 'duration'.  This was causing problems with old .sif files which use animated valuenodes for start_time and/or end_time, since we couldn't easily convert them to produce the same results with the new layer type.  This commit adds a new parameter to the timeloop layer to cause the layer to disable itself for non-positive durations, allowing compatibility with old .sif files.

git-svn-id: http://svn.voria.com/code@1528 1f10aa63-cdf2-0310-b900-c93c546f37ac