Add detection of container/video_codec/audio_codec compatibility for live file streaming or transcoding (#384)

* add forceMKV, forceHEVC config options
* drop audio stream instead of trying to transcode for ffmpeg unsupported/unknown audio codecs

pkg/ffmpeg/media_detection.go

@@ -0,0 +1,66 @@
+package ffmpeg
+
+import (
+	"bytes"
+	"github.com/stashapp/stash/pkg/logger"
+	"os"
+)
+
+// detect file format from magic file number
+// https://github.com/lex-r/filetype/blob/73c10ad714e3b8ecf5cd1564c882ed6d440d5c2d/matchers/video.go
+
+func mkv(buf []byte) bool {
+	return len(buf) > 3 &&
+		buf[0] == 0x1A && buf[1] == 0x45 &&
+		buf[2] == 0xDF && buf[3] == 0xA3 &&
+		containsMatroskaSignature(buf, []byte{'m', 'a', 't', 'r', 'o', 's', 'k', 'a'})
+}
+
+func webm(buf []byte) bool {
+	return len(buf) > 3 &&
+		buf[0] == 0x1A && buf[1] == 0x45 &&
+		buf[2] == 0xDF && buf[3] == 0xA3 &&
+		containsMatroskaSignature(buf, []byte{'w', 'e', 'b', 'm'})
+}
+
+func containsMatroskaSignature(buf, subType []byte) bool {
+	limit := 4096
+	if len(buf) < limit {
+		limit = len(buf)
+	}
+
+	index := bytes.Index(buf[:limit], subType)
+	if index < 3 {
+		return false
+	}
+
+	return buf[index-3] == 0x42 && buf[index-2] == 0x82
+}
+
+//returns container as string ("" on error or no match)
+//implements only mkv or webm as ffprobe can't distinguish between them
+//and not all browsers support mkv
+func MagicContainer(file_path string) Container {
+	file, err := os.Open(file_path)
+	if err != nil {
+		logger.Errorf("[magicfile] %v", err)
+		return ""
+	}
+
+	defer file.Close()
+
+	buf := make([]byte, 4096)
+	_, err = file.Read(buf)
+	if err != nil {
+		logger.Errorf("[magicfile] %v", err)
+		return ""
+	}
+
+	if webm(buf) {
+		return Webm
+	}
+	if mkv(buf) {
+		return Matroska
+	}
+	return ""
+}