[cache] Revise the dynamic hash table accounting.

Instead of counting entries relative to the middle of the hash table,
this switches to the absolute counter with the full index range mask.
As a result, some calculations become a bit simpler.  The cache resizing
logic stays largely the same.

* src/cache/ftccache.h (FTC_NODE_TOP_FOR_HASH): Revised with new counter.
* src/cache/ftccache.c (ftc_get_top_node_for_hash): Ditto.
(ftc_cache_resize): Simplify reallocations and stop their zeroing.
(ftc_cache_init): Stop over-allocating but keep zeroing initially.
(FTC_Cache_Clear, FTC_Cache_RemoveFaceID): Updated accordingly.