ffmpegdemux: cache events from upstream and re-send them later

Cache any events we get from upstream before we're open, especially
tag events we may be getting from apedemux/id3demux or the like, and
push them downstream later when we've added our pads instead of just
dropping them silently. Fixes transcoding tags for Monkey's Audio
Files with preceding APE or ID3v2 tags (#586957). Add minimal unit
test for this.

Also push stream tags later after the global tags and the newsegment
event rather than right after creating the pad.