Only return true from isReady after rendering.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=110231626

extensions/vp9/src/main/java/com/google/android/exoplayer/ext/vp9/LibvpxVideoTrackRenderer.java

@@ -376,7 +376,7 @@ public final class LibvpxVideoTrackRenderer extends SampleSourceTrackRenderer {
 
 
   @Override
   @Override
   protected boolean isReady() {
   protected boolean isReady() {
-    return format != null && (sourceIsReady || outputBuffer != null);
+    return format != null && (sourceIsReady || outputBuffer != null) && renderedFirstFrame;
   }
   }
 
 
   @Override
   @Override