OpenGL: Implement Fencing backend.

src/video_core/rasterizer_interface.h

@@ -49,6 +49,14 @@ public:
     /// Records a GPU query and caches it
     virtual void Query(GPUVAddr gpu_addr, QueryType type, std::optional<u64> timestamp) = 0;
 
+    virtual void SignalFence(GPUVAddr addr, u32 value) {
+
+    }
+
+    virtual void ReleaseFences() {
+
+    }
+
     /// Notify rasterizer that all caches should be flushed to Switch memory
     virtual void FlushAll() = 0;