util/disk_cache: compress individual cache entries

This reduces the cache size for Deus Ex from ~160M to ~30M for
radeonsi (these numbers differ from Grigori's results below
probably due to different graphics quality settings).

I'm also seeing the following improvements in minimum fps in the
Shadow of Mordor benchmark on an i5-6400 [email protected], with a HDD:

no-cache:                    ~10fps
with-cache-no-compression:   ~15fps
with-cache-and-compression:  ~20fps

Note: The with cache results are from the second run after closing
and opening the game to avoid the in-memory cache.

Since we mainly care about decompression I went with
Z_BEST_COMPRESSION as suggested on irc by Steinar H. Gunderson
who has benchmarked decompression speeds.

Grigori Goronzy provided the following stats for Deus Ex: Mankind
Divided start-up times on a Athlon X4 860k with a SSD:

No Cache                                 215 sec

Cold Cache zlib BEST_COMPRESSION         285 sec
Warm Cache zlib BEST_COMPRESSION         33 sec

Cold Cache zlib BEST_SPEED               264 sec
Warm Cache zlib BEST_SPEED               33 sec

Cold Cache no compression                266 sec
Warm Cache no compression                34 sec

The total cache size for that game is 48 MiB with BEST_COMPRESSION,
56 MiB with BEST_SPEED and 170 MiB with no compression.

These numbers suggest that it may be ok to go with Z_BEST_SPEED
but we should gather some actual decompression times before doing
so. Other options might be to do the compression in a separate
thread, this might allow us to use a higher compression algorithim
such as LZMA.

Reviewed-by: Grigori Goronzy <[email protected]>
Acked-by: Marek Olšák <[email protected]>

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