Samples - adding a sample to illustrate StreamProvider for demuxer (#99)

* mem sample

* changes to match c++ style

* ffmpeg log

* readme update

* readme clean up

utils/video_demuxer.h

@@ -152,6 +152,7 @@ bool VideoDemuxer::Demux(uint8_t **video, int *video_size, int64_t *pts) {
 }
 
 VideoDemuxer::VideoDemuxer(AVFormatContext *av_fmt_input_ctx) : av_fmt_input_ctx_(av_fmt_input_ctx) {
+    av_log_set_level(AV_LOG_QUIET);
     if (!av_fmt_input_ctx_) {
         std::cerr << "ERROR: av_fmt_input_ctx_ is not vaild!" << std::endl;
         return;
@@ -255,7 +256,6 @@ AVFormatContext *VideoDemuxer::CreateFmtContextUtil(StreamProvider *stream_provi
 
 AVFormatContext *VideoDemuxer::CreateFmtContextUtil(const char *input_file_path) {
     avformat_network_init();
-    av_log_set_level(AV_LOG_QUIET);
     AVFormatContext *ctx = nullptr;
     if (avformat_open_input(&ctx, input_file_path, nullptr, nullptr) != 0 ) {
         std::cerr << "ERROR: avformat_open_input failed!" << std::endl;