pad: don't accept flush-stop on inactive pads

pad: don't accept flush-stop on inactive pads

Inactive pads should at all times have the flushing flag set. This means
that when we get a flush-stop on an inactive pad we must ignore it.

On sinkpads, make this more explicit. We used to not clear the flush
flag but remove the events and then return an error because the flushing
flag was set. Now just simply refuse the event without doing anything.

On srcpads, check that we are trying to push a flush-stop event and
refuse it. We would allow this and mark the srcpad as non-flushing
anymore.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=735357