vtenc: free input buffer ASAP

VTCompressionSessionEncodeFrame retains the CVPixelBuffer during
encoding, and will release it as soon as it can (e.g. before it even
calls our callback). This means we can safely release input buffer
at this point, possibly allowing the system to reuse it sooner.

https://bugzilla.gnome.org/show_bug.cgi?id=750671

sys/applemedia/vtenc.c

@@ -1202,6 +1202,10 @@ gst_vtenc_encode_frame (GstVTEnc * self, GstVideoCodecFrame * frame)
         (int) vt_status);
   }
 
+  /* VTCompressionSessionEncodeFrame retained pbuf
+   * and we want to free input_buffer ASAP */
+  gst_buffer_replace (&frame->input_buffer, NULL);
+
   gst_video_codec_frame_unref (frame);
 
   CVPixelBufferRelease (pbuf);