[Flac] decode_position for SeekableStream

[Scot Thompson]

All,

Implemented SeekableStream::get_decode_position(.) as follows.  As you can
see it's just a C++ wrapper around the already existing get_decode_position
in the C version library.

Add the following to seekable_stream_decoder.cpp in libFLAC++:

bool SeekableStream::get_decode_position(FLAC__uint64 *position) const
	{
	FLAC__ASSERT(is_valid());
	return
(bool)::FLAC__seekable_stream_decoder_get_decode_position(decoder_,
position);
	}

Add the following to decoder.h in libFLAC++ (in the SeekableStream portion):

bool get_decode_position(FLAC__uint64* position) const;