Allow values for TrackNumber > 0; map TrackNumber to external index (from https://bugzilla.mozilla.org/show_bug.cgi?id=804372)

From Mozilla Bug 804372 Comment 7 (https://bugzilla.mozilla.org/show_bug.cgi?id=804372#c7)...

-- Allows values for TrackNumber other than 0 in ne_read_block.

-- Provides a new internal function ne_map_track_number_to_index to map TrackNumber to the external index/reference value. This index is then stored in the nestegg packet, which is used externally. The index should have the same value externally, with or without the patch (discounting the error cases this patch aims to fix).

I have tested it with single stream audio files, multiplexed audio/video files, and DASH audio/video files - all with at least one TrackNumber > TrackCount. All pass.