Fix global vars in LZWEncoder
Fix global vars in LZWEncoder

**Bug**
A few vars in LZWEncoder are globals, preventing strict mode from working.

I'm trying to use this library with Babel/webpack, which automatically tacks on "use strict".

**Fix**
Scope the 3 vars to the LZWEncoder class instead. I believe this was the intent.

twolfson/gif-encoder,twolfson/gif-encoder