Fifo: Make g_bSkipCurrentFrame a TU-local variable

This is only ever queried, making it a global isn't necessary.

Source/Core/VideoBackends/Software/EfbCopy.cpp

@@ -72,7 +72,7 @@ namespace EfbCopy
 		rc.right = rc.left + (int)bpmem.copyTexSrcWH.x + 1;
 		rc.bottom = rc.top + (int)bpmem.copyTexSrcWH.y + 1;
 
-		if (!Fifo::g_bSkipCurrentFrame)
+		if (!Fifo::WillSkipCurrentFrame())
 		{
 			if (bpmem.triggerEFBCopy.copy_to_xfb)
 			{