DRM mirror across two displays

[AlexHCC]

Simple example to make the two HDMI outputs on the pi 4 to mirror the DRM preview. (Didn't add overlay mirroring) A general implementation would be to get an array of all available crtcs from all DRM cards and create all all necessary planes for use in the atomic commit. Also different transforms for different monitors or a fullscreen option could be considered (I for example want fullscreen previews for all connected monitors regardless of their resolution).

https://github.com/raspberrypi/picamera2/issues/1073#issuecomment-2243081544