Start to implement eglstream-kms DisplayBuffer logical layout:
* Use the logical size (extents) of the output, which takes into
consideration transformations (presently orientation, later others).
* Return the correct transformation from DisplayBuffer::transformation()
so that the renderer knows what to do. Fixes: https://bugs.launchpad.net/bugs/1639226.
Approved by Chris Halse Rogers, Kevin DuBois, mir-ci-bot.