No more reference counts for GIG file instances

Since libgig can't really be used in a multithreaded way unless it was
somewhat rewritten, just use a separate instance of the file for each
new GIG file regardless of if we already have one open in the current
file. Since it's fast now, you can easily have quite a few very large
GIG files open and still have low latency.

Also removed C++11 requirement since I no longer need a move
constructor.