VideoCommon/Statistics: Remove global system accessor from s_after_frame_event

Instead, we make the event take a reference to the system and then pass
it in when the event is triggered.

This does introduce two other accessors, but these are much easier to
refactor out over time, and without modification to the existing event
interface.

Source/Core/VideoCommon/BPStructs.cpp

@@ -346,7 +346,7 @@ static void BPWritten(PixelShaderManager& pixel_shader_manager, XFStateManager&
       // render multiple sub-frames and arrange the XFB copies in next to each-other in main memory
       // so they form a single completed XFB.
       // See https://dolphin-emu.org/blog/2017/11/19/hybridxfb/ for examples and more detail.
-      AfterFrameEvent::Trigger();
+      AfterFrameEvent::Trigger(Core::System::GetInstance());
 
       // Note: Theoretically, in the future we could track the VI configuration and try to detect
       //       when an XFB is the last XFB copy of a frame. Not only would we get a clean "end of