Move MetaOutput::crtc field into private struct and add ref-chain

I'm reproposing here (for easier reviewing) the patches that @jadahl submitted for bug 786929 rebased on current mutter fixing the conflicts.

First commit was actually marked as commit-now, but still has a change on the way we handle the transformation that should be double-checked.

Previous reviews:

• backends: Move MetaOutput::crtc field into private struct
• backends: Add logical monitor -> monitor -> output -> crtc ref chain