add proper support for GXFIFO stalls.

bad games that blast the GXFIFO and overflow it:
* Super Mario 64 DS
* Rayman RR2

latter seems to get its music streaming crapoed.

src/GPU3D.h

@@ -90,6 +90,7 @@ void DoSavestate(Savestate* file);
 
 void ExecuteCommand();
 
+s32 CyclesToRunFor();
 void Run(s32 cycles);
 void CheckFIFOIRQ();
 void CheckFIFODMA();