Change creat()/write()/close() to fopen()/fwrite()/fclose()

[hsnyder]

You may not be interested in these changes, which is fine of course, but I implemented these for my own purposes and I figured that I would offer them back.

This PR changes 3 things:

• creat()/write()/close() was replaced with fopen()/fwrite()/fclose(). My reason for implementing this was in case the user wanted to write to a buffer created with fmemopen instead of to an actual file (I have such a use case).

• A new function was added, ge_new_gif_filestream that accepts a FILE* instead of a filename.

• The function ge_close_gif now returns the total length of the written gif file as a size_t