Fifo: Make g_bSkipCurrentFrame a TU-local variable

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

Source/Core/VideoCommon/BPStructs.cpp

@@ -380,7 +380,7 @@ static void BPWritten(const BPCmd& bp)
 	case BPMEM_CLEARBBOX2:
 		// Don't compute bounding box if this frame is being skipped!
 		// Wrong but valid values are better than bogus values...
-		if (!Fifo::g_bSkipCurrentFrame)
+		if (!Fifo::WillSkipCurrentFrame())
 		{
 			u8 offset = bp.address & 2;
 			BoundingBox::active = true;