make textures_dump_atlases work again and expose it as a utility function

src/twn_textures_c.h

@@ -50,9 +50,6 @@ typedef struct TextureKey { uint16_t id; } TextureKey;
 void textures_cache_init(struct TextureCache *cache, SDL_Window *window);
 void textures_cache_deinit(struct TextureCache *cache);
 
-/* for debugging */
-void textures_dump_atlases(struct TextureCache *cache);
-
 /* loads an image if it isn't in the cache, otherwise a no-op. */
 /* can be called from anywhere at any time after init, useful if you want to */
 /* preload textures you know will definitely be used */