implement webm to ogg demuxer

* used for opus audio stream
* update WebMReader and WebMWriter
* new post-processing algorithm

app/src/main/java/us/shandian/giga/postprocessing/OggFromWebmDemuxer.java

@@ -0,0 +1,44 @@
+package us.shandian.giga.postprocessing;
+
+import android.support.annotation.NonNull;
+
+import org.schabi.newpipe.streams.OggFromWebMWriter;
+import org.schabi.newpipe.streams.io.SharpStream;
+
+import java.io.IOException;
+import java.nio.ByteBuffer;
+
+class OggFromWebmDemuxer extends Postprocessing {
+
+    OggFromWebmDemuxer() {
+        super(false, true, ALGORITHM_OGG_FROM_WEBM_DEMUXER);
+    }
+
+    @Override
+    boolean test(SharpStream... sources) throws IOException {
+        ByteBuffer buffer = ByteBuffer.allocate(4);
+        sources[0].read(buffer.array());
+
+        // youtube uses WebM as container, but the file extension (format suffix) is "*.opus"
+        // check if the file is a webm/mkv file before proceed
+
+        switch (buffer.getInt()) {
+            case 0x1a45dfa3:
+                return true;// webm
+            case 0x4F676753:
+                return false;// ogg
+        }
+
+        throw new UnsupportedOperationException("file not recognized, failed to demux the audio stream");
+    }
+
+    @Override
+    int process(SharpStream out, @NonNull SharpStream... sources) throws IOException {
+        OggFromWebMWriter demuxer = new OggFromWebMWriter(sources[0], out);
+        demuxer.parseSource();
+        demuxer.selectTrack(0);
+        demuxer.build();
+
+        return OK_RESULT;
+    }
+}