Fix memory leaks in tile loading

Fix memory leaks in tile loading

- Splitting a tile could accidentally overwrite a already split tile if
  one (but not all) of the children were garbage collected.

- Garbage collection never actually happened.. because tile always had
  empty children. To fix this a `load' flag was added so that the
  children could be garbage collected as well without having to check
  the data flag which may always be null.

tile->load: set when texture loading has started
tile->data: set when texture loading has finished