mpegtsmux: Improve PCR/SI scheduling.

mpegtsmux: Improve PCR/SI scheduling.

Change PCR / SI scheduling so that instead of checking if
the current PCR is larger than the next target time, instead
check if the PCR of the next packet would be too late, so PCR
and SI are always scheduled earlier than the target, not later.

There are still cases where PCR can be written too late though,
because we don't check before each output packet.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2073>