This adds about a frame of latency, and since most games don't change
VI registers during scanout, we can get away with outputting the XFB at
the start of scanout. WWE Crush Hour is the (only currently known)
exception, which has flickering problems when doing it this way.
This adds a path to perform the output at the end of scanout, and gates
it behind an option which defaults to using the latency-reducing
pre-scanout path.
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