Rewite almost code.
- change avisynth_c.h to Avisynth+MT's avisynth.h.
- change C99 to C++11.
- change mingw-w64 to VisualC++2015.
- drop Avisynth2.5x support.
- drop WindowsXP support.
- drop x264bd and x264raw options.
- add Avisynth+MT's new HBD formats support.
- add FFmpeg's extended Y4M formats output support.