When the application is put into background on iOS, the VideoToolbox session
is boken. That resulted in the VideoToolbox filter was not able to decode
any frame once the application has been resumed.

That fix destroys the decoding context when the -12903 status code
is returned by the VTDecompressionSessionDecodeFrame function.
Then, the decompression session will be created again when a new frame
is recieved.