WIP: clients: add simple-dmabuf-gbm client

Add a "weston-simple-dmabuf-gbm" test client which uses GBM for its buffer management, and is designed to support a wide variety of image types.

I've only tested this with the i915 (gen4 GPU) and vc4 backend drivers.

[See also the commit message.]

With 1300 lines added, there will be formatting issues, and bugs.

WIP: because CI is failing, due to a version mismatch with libgbm from Mesa, and misconfigured drm_fourcc.h dependency.