Use simplified GIF interface

[mdales]

This PR does three things:

1. I simplified the API on giflib to prevent users having to do the LZW compression themselves, and this PR changes Claudius to use that thus keeping Claudius code more clean.
2. In reading the GIFlib code I realise that GIFlib will pad the ColorTable.t to the nearest power of two, which Claudius was also doing, and so again I opted to simplify the Claudius code. What we did before wasn't wrong, but just might as well not have two things trying to do the same thing.
3. In reading the GIFlib code I realise that we should set the GIF image delay time value to match the Cladius frame rate as best we can - before animations looked a little slow when viewed, and this is why.

[pawaskar-shreya]

In reading the GIFlib code I realise that GIFlib will pad the ColorTable.t to the nearest power of two

Yeaa, we had abstracted this piece of code from both the animation.ml and screenshot.ml during the screenshot feature to utils.ml

In reading the GIFlib code I realise that we should set the GIF image delay time value to match the Cladius frame rate as best we can - before animations looked a little slow when viewed, and this is why.

Yeaaaa, I had noticed this tooo! But couldn't really figure out the reason as to why that was happening. Now I can understnad the why behind it!