GPU: Add virtual call when buffers are swapped

src/core/gpu.cpp

@@ -1291,6 +1291,7 @@ void GPU::WriteGP1(u32 value)
         SynchronizeCRTC();
         m_crtc_state.regs.display_address_start = new_value;
         UpdateCRTCDisplayParameters();
+        OnBufferSwapped();
       }
     }
     break;
@@ -2833,6 +2834,10 @@ void GPU::DrawRendererStats()
 {
 }
 
+void GPU::OnBufferSwapped()
+{
+}
+
 void GPU::GetStatsString(SmallStringBase& str)
 {
   if (IsHardwareRenderer())