With the addition of the changeType API for MediaSource, it is theoretically possible for a variant to change between multiple codecs for a given buffer, over the course of playback.
This adds support for the DASH player to stitch together periods which have such multi-codec variants, but only as a last resort. For example, if one period only has audio in aac, and another period only has opus audio, the player will now stitch those periods together as one, but if there is a throughline that does not involve changing codecs it will go for that instead.
Closes#5961
theodab
24e32559bf