textures.c: remove by one offsetiing, use macro for validity checking

src/textures.h

@@ -11,10 +11,7 @@
 typedef struct { int id; } t_texture_key;
 
 /* tests whether given key structure corresponds to any texture */
-#define m_texture_key_is_valid(p_key) ((p_key).id != 0)
-
-/* tests whether given key corresponds to atlas texture or a loner */
-#define m_texture_is_atlas(p_key) ((p_key).id > 0)
+#define m_texture_key_is_valid(p_key) ((p_key).id != -1)
 
 void textures_cache_init(struct texture_cache *cache, SDL_Window *window);
 void textures_cache_deinit(struct texture_cache *cache);