If a screen layout cannot be applied, instead of failing to start
WindowServer try to fall back to an auto-generated screen layout with
the devices that are detected.
Also, be a bit smarter about changing the current screen layout.
Instead of closing all framebuffers and bringing them back up, keep
what we can and only change resolution on those that we need to change
them on. To make this work we also need to move away from using an
array of structures to hold compositor related per-screen data to
attaching it to the Screen itself, which makes re-using a screen much
simpler.
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