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/VideoBackendBase.h

@@ -52,7 +52,7 @@ public:
 
   void Video_ExitLoop();
 
-  void Video_BeginField(u32 xfb_addr, u32 fb_width, u32 fb_stride, u32 fb_height, u64 ticks);
+  void Video_OutputXFB(u32 xfb_addr, u32 fb_width, u32 fb_stride, u32 fb_height, u64 ticks);
 
   u32 Video_AccessEFB(EFBAccessType type, u32 x, u32 y, u32 data);
   u32 Video_GetQueryResult(PerfQueryType type);