Reuse the test decoder to improve startup performance

app/streaming/video/ffmpeg-renderers/renderer.h

@@ -190,6 +190,9 @@ public:
         // If the renderer doesn't provide an explicit test routine,
         // we will always assume that any returned AVFrame can be
         // rendered successfully.
+        //
+        // NB: The test frame passed to this callback may differ in
+        // dimensions from the actual video stream.
         return true;
     }