dashdemux: segmentbase type with 'sidx' is not working as expected.

Submitted by Jun Xie

Description

e.g.
http://dash.edgesuite.net/dash264/TestCases/1a/netflix/exMPD_BIP_TC1.mpd

currently, a whole file is downloaded without using range download, and also bitrate switch is not available.
The expected behaviour shall be that 'sidx' parsed,
and segments be retrieved by range download, and bitrate can be switched.