DolphinWX: Allow cancelling gcz compression.
DiscIO: Decompress to file using a larger buffer.

This improves performance by around 20% for me, and the memory use impact
is negligible considering Dolphin is otherwise unusable.
DiscIO: Avoid zeroing buffer when compressing gcz.

This saves 6% time.
DiscIO: Change a memset() to zero initialization.
Merge pull request #1597 from unknownbrackets/gcz-tweaks

Tweaks to gcz compression / decompression