Tweaks to gcz compression / decompression

This makes it so you can cancel either process, it does after all take quite some time.  It also improves the speed of compression and decompression.

I've also found that std::fill(), at least via MSVC, is significantly slower than memset() (where equivalent.)  It seems like changing this one would give another 6% gain for Wii isos:
https://github.com/dolphin-emu/dolphin/blob/master/Source/Core/DiscIO/DiscScrubber.cpp#L135

-[Unknown]