LibVideo: Propagate decoder errors in the Matroska Reader

Matroska::Reader functions now return DecoderErrorOr instead of values
being declared Optional. Useful errors can be handled by the users of
the parser, similarly to the VP9 decoder. A lot of the error checking
in the reader is a lot cleaner thanks to this change, since all reads
can be range checked in Streamer::read_octet() now.

Most functions for the Streamer class are now also out-of-line in
Reader.cpp now instead of residing in the header.

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

@@ -20,7 +20,7 @@ public:
     static DecoderErrorOr<NonnullOwnPtr<MatroskaDemuxer>> from_file(StringView filename);
     static DecoderErrorOr<NonnullOwnPtr<MatroskaDemuxer>> from_data(ReadonlyBytes data);
 
-    MatroskaDemuxer(NonnullOwnPtr<MatroskaDocument>& document)
+    MatroskaDemuxer(NonnullOwnPtr<MatroskaDocument>&& document)
         : m_document(move(document))
     {
     }