cache sprite srcrects

src/twn_textures_c.h

@@ -50,6 +50,7 @@ typedef struct TextureCache {
 typedef struct TextureKey { uint16_t id; } TextureKey;
 
 /* tests whether given key structure corresponds to any texture */
+#define TEXTURE_KEY_INVALID (TextureKey) { (uint16_t)-1 }
 #define m_texture_key_is_valid(p_key) ((p_key).id != (uint16_t)-1)
 
 void textures_cache_init(struct TextureCache *cache, SDL_Window *window);