release version 0.4.1

• macOS: fix glx weak link
• support ubuntu >= 14.04
• load vaapi dynamically
• improve vaapi host map, control via env var VAIMAGE_DERIVE, VAIMAGE_FORMAT
• fix many playback loop bugs when more tracks are enabled
• fix too many packets buffered when decoding the 1st frame
• MFT: disable low latency to fix frames out of order
• subtitle support WIP.
  • decode embedded subtitle tracks via avcodec
  • decode loop and thread
• FFmpeg
  • versioned dso is preferred
  • reload ffmpeg symbols if dso is changed by SetGlobalOption()
  • log level is controlled by SetGlobalOption("ffmpeg.loglevel", val), val can be "quiet", "panic", "fatal", "error", "warning", "info", "verbose", "debug", "trace"