contrib/libaribcaption: Update library version and improve performance

Hi, I'm the author and maintainer of the libaribcaption project and codec/arib/libaribcaption.c module.

Since I have released several versions for libaribcaption, it's time to download the tarball file of a specific version, rather than pulling the entire git repository. I changed the rules.mak to download libaribcaption-1.1.1.tar.gz from github.

In addition, I added some logic to detect whether the caption images are changed, and skip the rendering call if unchanged to avoid the overhead of underlying repeated memcpy occurring per frame.

I've tested these changes and it works as expected.

Regards