Add Debian bug reference for libcaca issues

Tracking these in one single Debian BTS bug as the affected version is
across all suites present and upstream fixed those issues with a set of
commits.

Covering as well the unimportant issues which do not affect the binary
built itself (Debian binary packages built with the Imlib2 library) with
this tracking bug.

Expecting upstream (and the Debian maintainer) will just fix all those
issues together via unstable either cherry-picking the set of commits or
releasing a new upstream 0.99.beta20 version.

Further investigation pending if they warrant a DSA for stable.