rtpfunnel: forward correct segment when switching pad

Forwarding a single segment event from the pad that first gets
chained is incorrect: when that first event was sent by an element
such as x264enc, with its offset start, we end pushing out of segment
buffers for the other pad(s).

Instead, everytime the active pad changes, forward the appropriate
segment event.

Fixes gstreamer/gst-plugins-bad#1028
18 jobs for backport-mr-281-into-1.16-rtpfunnel-fix-segment in 92 minutes and 45 seconds (queued for 54 seconds)
Status Job ID Name Coverage
  Preparation
passed #488326
gst indent

00:00:55

passed #488325
manifest

00:00:59

 
  Build
passed #488332
build cerbero android universal

00:17:23

passed #488333
build cerbero cross win32

00:09:32

passed #488334
build cerbero cross win64

00:07:36

passed #488331
build cerbero fedora x86_64

00:05:10

passed #488336
gst-ios-12.2
build cerbero ios universal

00:44:22

passed #488335
gst-macos-10.14
build cerbero macos x86_64

00:08:23

passed #488327
build fedora x86_64

00:08:15

passed #488328
build nodebug fedora x86_64

00:07:25

passed #488329
build static fedora x86_64

00:13:13

passed #488330
build static nodebug fedora x86_64

00:10:01

 
  Test
passed #488337
check fedora

00:03:25

passed #488338
integration testsuites fedora

00:16:17

passed #488340
valgrind bad

00:06:39

passed #488341
valgrind ges

00:11:02

passed #488339
valgrind good

00:10:22

 
  Integrate
passed #488342
android universal examples

00:09:36