tests: support mp4/avcC from avformat for h264

'yamidecode -m 0/-1' works, however, there are strange exception for other video rendering
- call XInitThreads() at the begining
- init m_convert to NULL for DecodeOutputRaw
- add IS_AVCC flag for decoder input buffer
- set default avcC size field to 4 bytes
tests: expose strange x11 exception when avformat is used

when we do not use rendering with X11 (-m 0/-1), there is no such issue

the debug code can be removed when we find the root issue and fixed it;
otherwise there will still be similar exception from vaapi driver

    libyami debug(decodeoutput.cpp, 305): m_window=0xb60060
    X Error of failed request:  BadWindow (invalid Window parameter)
      Major opcode of failed request:  3 (X_GetWindowAttributes)
      Resource id in failed request:  0xb60060
      Serial number of failed request:  14
      Current serial number in output stream:  15
tests: refine for net fps calculation

renderOutputFrames() may renders more than one frame