Fixes#286.
I dug into Firefox code a bit to understand this but got a lost. But
I guess there are different policies for what's accessible via
`video.src = ""` than for `<video src="">`. This works for now, matches
what other players such as `mpegts.js` do, and is closer to what Safari
MME (required on iPhone) needs anyway. (With MME, apparently you have to
use `video.srcObject`, or the `MediaSource`'s `sourceopen` event will
never fire.)