Fix clip/tilesheet height sanity check
Update CMake plugin directory logic

Plugin directory should always have been PATH instead of FILEPATH, and
should have been protected against the possibility that the default may
not exist. Users also now have the ability to disable the -copy target
by leaving the field blank.
Add Avisynth64 support

See http://forum.doom9.org/showthread.php?t=152800 or
https://code.google.com/p/avisynth64/ for details. The project is long
dead, but it does technically work, and my plugin compiles with only
marginal changes, so I thought I'd throw this in.