Previously, if the presentation delay was not specified either by the
manifest or by configuration, we defaulted to 1.5 * minBufferTime. That
approach worked for most content, but in some cases it could cause live
streams to start out with a seekRangeEnd that was before the live edge.
This would lead to a brief pause in the beginning of the presentation,
while the live edge caught up.
This changes the DASH parser to use the segmentAvailabilityDuration if
it is lower than 1.5 * minBufferTime, which fixes that issue.
It also changes how that part of the code to be formatted, in order to
hopefully make the increasingly-complex logic for determining the
presentation delay more clear.
theodab
b9eabe5441