Use ne_map_track_number_to_index in nestegg_track_seek (from https://bugzilla.mozilla.org/show_bug.cgi?id=792935)

A track_number to track_id mapping was introduced in Mozilla Bug 804372 (https://bugzilla.mozilla.org/show_bug.cgi?id=804372) for nestegg. The patch did not update nestegg_track_seek (similar to Mozilla Bug 832018 - https://bugzilla.mozilla.org/show_bug.cgi?id=832018), so the WebM files with track numbers other than 1 do not work in DASH.

Note: I also searched through nestegg.c to find other occurrences of this - it looks like the last one.