Add basic loader for gltf

Added loading of avocado. Something is still wrong though,
becaus I cannot see the seed. But basic functionality is there.

utilities/commons.h

@@ -28,7 +28,7 @@
         exit(1);                                                                    \
     }
 
-#define rse_free(ptr) free(ptr);
+#define rse_free(ptr) if (ptr != NULL) free(ptr);
 
 #define rse_memcpy(dst, src, size) memcpy(dst, src, size);
 #define rse_memset(ptr, value, size) memset(ptr, value, size);