VideoCommon: Use XFB for internal resolution stats

fb4ff3e added a statistics option to show the internal resolution, but
it just showed the total size of the EFB (which is always 640x528) times
the IR scale, so it didn't convey any useful information.

This commit instead makes the option use the size of the last XFB copy
(not multiplied by the IR scale), which changes based on the game's
rendering resolution.

Source/Core/Core/Core.cpp

@@ -863,6 +863,9 @@ void Callback_FramePresented(const PresentInfo& present_info)
       present_info.actual_present_time - present_info.intended_present_time;
   perf_metrics.SetLatestFramePresentationOffset(presentation_offset);
 
+  perf_metrics.SetLatestFrameBufferSize(present_info.frame_buffer_width,
+                                        present_info.frame_buffer_height);
+
   if (present_info.reason == PresentInfo::PresentReason::VideoInterfaceDuplicate)
     return;