WIP: gitlab-ci: add clang-format config and set it up
Copy the clang-format config from virgl, tweaking it slightly - column sizes and include files order.
There are a couple of small changes through the codebase, but those can be updated as the respective code is touched - as enforced by the new gitlab-ci step.
Note: we need to add clang-format-7 and git explicitly, since the former Debian package is missing the latter as dependency.
WIP: Need to polish the documentation and reference