Cleanup: use '_len' instead of '_size' w/ BLI API

- When returning the number of items in a collection use BLI_*_len()
- Keep _size() for size in bytes.
- Keep _count() for data structures that don't store length
  (hint this isn't a simple getter).

See P611 to apply instead of manually resolving conflicts.

source/blender/imbuf/intern/moviecache.c

@@ -513,7 +513,7 @@ void IMB_moviecache_get_cache_segments(MovieCache *cache, int proxy, int render_
 		*points_r = cache->points;
 	}
 	else {
-		int totframe = BLI_ghash_size(cache->hash);
+		int totframe = BLI_ghash_len(cache->hash);
 		int *frames = MEM_callocN(totframe * sizeof(int), "movieclip cache frames");
 		int a, totseg = 0;
 		GHashIterator gh_iter;