refactor(libvideo2x): extracted video frame count estimation into a function

Signed-off-by: k4yt3x <i@k4yt3x.com>

include/libvideo2x/avutils.h

@@ -0,0 +1,10 @@
+#ifndef AVUTILS_H
+#define AVUTILS_H
+
+extern "C" {
+#include <libavformat/avformat.h>
+}
+
+int64_t get_video_frame_count(AVFormatContext *ifmt_ctx, int in_vstream_idx);
+
+#endif  // AVUTILS_H