LibVideo: Reorganize demuxer file hierarchy and rename Matroska files

As new demuxers are added, this will get quite full of files, so it'll
be good to have a separate folder for these.

To avoid too many chained namespaces, the Containers subdirectory is
not also a namespace, but the Matroska folder is for the sake of
separating the multiple classes for parsed information entering the
Video namespace.

Userland/Libraries/LibVideo/Containers/Matroska/MatroskaDemuxer.h

@@ -0,0 +1,51 @@
+/*
+ * Copyright (c) 2022, Gregory Bertilson <zaggy1024@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/HashMap.h>
+#include <LibVideo/Containers/Demuxer.h>
+
+#include "Reader.h"
+
+namespace Video::Matroska {
+
+class MatroskaDemuxer final : public Demuxer {
+public:
+    // FIXME: We should instead accept some abstract data streaming type so that the demuxer
+    //        can work with non-contiguous data.
+    static DecoderErrorOr<NonnullOwnPtr<MatroskaDemuxer>> from_file(StringView filename);
+    static DecoderErrorOr<NonnullOwnPtr<MatroskaDemuxer>> from_data(ReadonlyBytes data);
+
+    MatroskaDemuxer(NonnullOwnPtr<MatroskaDocument>& document)
+        : m_document(move(document))
+    {
+    }
+
+    Vector<Track> get_tracks_for_type(TrackType type) override;
+
+    DecoderErrorOr<void> seek_to_most_recent_keyframe(Track track, size_t timestamp) override;
+
+    Time duration() override;
+
+protected:
+    DecoderErrorOr<NonnullOwnPtr<Sample>> get_next_sample_for_track(Track track) override;
+
+private:
+    struct TrackStatus {
+        size_t m_cluster_index { 0 };
+        size_t m_block_index { 0 };
+        size_t m_frame_index { 0 };
+    };
+
+    ErrorOr<TrackStatus*> get_track_status(Track track);
+
+    NonnullOwnPtr<MatroskaDocument> m_document;
+
+    HashMap<Track, TrackStatus> m_track_statuses;
+};
+
+}