util/disk_cache: Support combined foz ro and non-foz rw caches

Mesa utilizes only one type of cache at a time. This patch enables support
for combined reading from read-only Fossilize cache + non-foz read-write
caches.

From now on, a non-foz read-write caches will first try to retrieve data
from a read-only foz cache if new MESA_DISK_CACHE_COMBINE_RW_WITH_RO_FOZ
environment variable is set to true, otherwise the caching behaviour is
unchanged. The new flag has no effect when MESA_DISK_CACHE_SINGLE_FILE=1,
i.e. when the single-file foz cache is used.

This change allows us to ship a prebuilt RO caches for a certain
applications, while the rest of applications will benefit from the
regular RW caching that supports cache-size limitation. This feature
will be used by ChromeOS.

Usage example #1:

MESA_DISK_CACHE_DATABASE=0
MESA_DISK_CACHE_SINGLE_FILE=0
MESA_DISK_CACHE_COMBINE_RW_WITH_RO_FOZ=1
MESA_DISK_CACHE_READ_ONLY_FOZ_DBS=rocache1,rocache2

Usage example #2:

MESA_DISK_CACHE_DATABASE=1
MESA_DISK_CACHE_SINGLE_FILE=0
MESA_DISK_CACHE_COMBINE_RW_WITH_RO_FOZ=1
MESA_DISK_CACHE_READ_ONLY_FOZ_DBS=rocache1,rocache2

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18551>

src/util/disk_cache_os.h

@@ -99,6 +99,9 @@ struct disk_cache {
       unsigned hits;
       unsigned misses;
    } stats;
+
+   /* Internal RO FOZ cache for combined use of RO and RW caches. */
+   struct disk_cache *foz_ro_cache;
 };
 
 struct cache_entry_file_data {