Incremental writing

Streaming writer. Allows outputting GIF immediately and doesn't require keeping all frames in memory at once. 

User can create each frame and free/recycle memory before writing next frame, so almost infinite gif streams are possible with constant memory usage.

```c
Gif_Writer *grr = Gif_WriteStart(gfs, gcinfo, fp, 1);
for(lots) {
    Gif_WriteImage(gfs, grr, gfi);
}
Gif_WriteEnd(gfs, grr);
```