Simple client to test the dma-buf feedback implementation. This does not
replace the need to implement a dma-buf feedback test that can be run in
the CI. But as we still don't know exactly how to do this, this client
can be helpful to run tests manually.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>