code: public void add(@NonNull Runnable command) {
          synchronized (mBufferLock) {
              if (mCurrentBuffer == null) {
                  mContext.runOnGlThread(command);
              } else {
                  mCurrentBuffer.add(command);
              }
          }
      }
docs: Add a {@link Runnable} to the current buffer.  If no buffer has been {@linkplain #start()
  started}, {@code command} will be {@linkplain GVRContext#runOnGlThread(Runnable) posted}
  directly to the GL thread to be executed in the next frame.
  
  @param command The command to add to the buffer.  Should be non-null.