Do not try to delay opening newly required playlists until a segment
switch. Applications expect that newly selected undiscarded streams are
available immediately, especially with alternative rendition streams
(selectable audio/subtitle tracks).

One might think that delaying variant stream switch until a segment
switch would allow a "seamless" switch without us having to download a
specific segment from two different variant playlists. However, that is
not the case, since the application would have to keep the previous
stream available (undiscarded) until the first packet of the newly
selected stream arrives, but by that time the demuxer would have already
downloaded the next segment of both variants.
Signed-off-by: Anssi Hannula <anssi.hannula@iki.fi>