Bundling of GLSLANG and SPIR-V

The source includes debian/get-external.sh script, which is used
to bundle new snapshot of GLSLANG and SPIR-V in the source. These
are required to build libVkLayer.so.

The procedure to update to a new upstream release goes usually like
this:

# git checkout upstream-unstable
# git reset --hard <new upstream tag>
# git checkout -b d-u
# git merge -s ours debian-unstable
# debian/get-external.sh
# dch
# debian/rules gentarball
