VI: Implement post-scanout XFB output

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.

Source/Core/VideoCommon/Fifo.cpp

@@ -373,7 +373,7 @@ void RunGpuLoop()
 
             // This call is pretty important in DualCore mode and must be called in the FIFO Loop.
             // If we don't, s_swapRequested or s_efbAccessRequested won't be set to false
-            // leading the CPU thread to wait in Video_BeginField or Video_AccessEFB thus slowing
+            // leading the CPU thread to wait in Video_OutputXFB or Video_AccessEFB thus slowing
             // things down.
             AsyncRequests::GetInstance()->PullEvents();
           }