更新libav.vcxproj, 禁用windows crt安全警告.

Signed-off-by: Jack <jack.wgm@gmail.com>
添加:
  EXPORT_API demux_context* alloc_demux_context();
  EXPORT_API void free_demux_context(demux_context *ctx);
更新:
  alloc_avplay_context
  alloc_audio_render
  alloc_video_render

Signed-off-by: Jack <jack.wgm@gmail.com>
fixed read_play&read_pause param name

Signed-off-by: Jack <jack.wgm@gmail.com>
添加备注信息.

Signed-off-by: Jack <jack.wgm@gmail.com>
添加MEDIA_DEMUX类型.

Signed-off-by: Jack <jack.wgm@gmail.com>