Allow setting cache size based on memory percentage (#184)

The problem:

1. For the most people imlib2's default 4MiB is unreasonably low;

2. Hardcoding cache size to ~256MiB has performance benefits and doesn't
   increase RAM usage too much on relatively modern systems;

3. But we can't do that, because that would be detrimental to low spec systems
   that (apparently) not (?) building nsxiv from source, as been discussed
   #171

Solution:

Calculate cache size based on total memory.

Default is set as 3%, which means:
* ~245MiB for 8GiB
* ~30MiB for 1GiB
* and so on

CACHE_SIZE_LIMIT (256MiB by default) sets the highest possible value. And in
case we cannot determine the total memory (e.g since _SC_PHYS_PAGES isn't
POSIX) use CACHE_SIZE_FALLBACK (32MiB by default) instead.

Co-authored-by: NRK <nrk@disroot.org>

config.def.h

@@ -62,11 +62,14 @@ static const bool ANTI_ALIAS = true;
  */
 static const bool ALPHA_LAYER = false;
 
-/* cache size for imlib2, in bytes. For backwards compatibility reasons, the
- * size is kept at 4MiB. For most users, it is advised to pick a value close to
- * or above 128MiB for better image (re)loading performance.
+/* percentage of memory to use for imlib2's cache size.
+ *   3 means use 3% of total memory which is about 245MiB on 8GiB machine.
+ *   0 or less means disable cache.
+ * 100 means use all available memory (but not above CACHE_SIZE_LIMIT).
  */
-static const int CACHE_SIZE = 4 * 1024 * 1024; /* 4MiB */
+static const int CACHE_SIZE_MEM_PERCENTAGE = 3;          /* use 3% of total memory for cache */
+static const int CACHE_SIZE_LIMIT = 256 * 1024 * 1024;   /* but not above 256MiB */
+static const int CACHE_SIZE_FALLBACK = 32 * 1024 * 1024; /* fallback to 32MiB if we can't determine total memory */
 
 #endif
 #ifdef _THUMBS_CONFIG