support strict mode

Out of the blue, avc.js began failing on me, with error messages about clip().

While debugging the problem I moved all global vars and functions into the Avc() function to avoid name clashes.
I then continued to support 'use strict' by removing the use of CONST and by moving the originals of patched functions into their own namespace instead of the global scope.

Sorry for the large and incoherent patch, I can break it up into (non-working) parts, if you prefer.