Avoid the test frame for DXVA2 and VT APIs to address flickering in full-screen on Win7

app/streaming/video/ffmpeg-renderers/vt.mm

@@ -189,6 +189,12 @@ public:
         return true;
     }
 
+    virtual bool needsTestFrame() override
+    {
+        // We query VT to determine whether the codec is supported
+        return false;
+    }
+
 private:
     void setupDisplayLayer()
     {