splitmuxsink: Does not request keyframes correctly if the only keyframes are the requested ones

See for example

gst-launch-1.0 -m videotestsrc num-buffers=350 ! video/x-raw, format=I420, width=320, height=240, framerate=25/1 ! x264enc ! hlssink2 target-duration=4

This needs !210 (merged) to work in any useful way at all.

The problem here is that a keyframe is requested for 4s. Then at 4s there is the keyframe but that does not overflow the current fragment. Then 10s later there happens to be another keyframe by accident, and that overflows so the first fragment is pushed out and directly followed by the second fragment (10s long).

The only code that requests no keyframes is in the very beginning, and then whenever an overflow is detected on a keyframe. But the overflow might already be too late, like in this case.

Unclear how to solve that properly. There seems to be no good place to add further keyframe requests currently.

CC @thaytan