Last segment indicated by mpd is not parsed.
Example stream: http://dash.akamaized.net/dash264/TestCasesIOP41/LastSegmentNumber/1/manifest_last_segment_num.mpd

This patch supports parsing of Supplemental Descriptor with @schemeIdUri set to http://dashif.org/guide-
lines/last-segment-number with the @value set to the last segment number.
Reviewed-by: Steven Liu <lq@onvideo.cn>