backends: Move MetaOutput::crtc field into private struct
No functional changes. This is only done so that changes to reference
counting can done more reliably.
Bug: https://bugzilla.gnome.org/show_bug.cgi?id=786929
Bug-Ubuntu: https://bugs.launchpad.net/ubuntu/+source/mutter/+bug/
1703668
Forwarded: yes, https://gitlab.gnome.org/GNOME/mutter/merge_requests/81
Applied-upstream: 3.28.3, commit:
7d01aec48da979f4fdad3c5f586fd0fd340d9325
Gbp-Pq: Name backends-Move-MetaOutput-crtc-field-into-private-struct.patch