rtsp-media: Add/configure transports when completing the pipeline

Otherwise the transports are not set up yet during the PLAY request
handling when unsuspending (and thus unblocking) the media.

In case of live pipelines this then causes the first few packets to go
to the sinks before they know what to do with them, and they simply
discard them which is rather suboptimal in case of keyframes.

For non-live pipelines this is not a problem because the sink will still
be PAUSED and as such not send out the data yet but wait until it goes
to PLAYING, which is late enough.

Adding the transports multiple times is not a problem: if the transport
is already added it won't be added another time and TRUE will be
returned.

This fixes a regression introduced by a7732a68
before 1.14.0.

Fixes gstreamer/gst-rtsp-server#107

Part-of: <gstreamer/gst-rtsp-server!135>
26 jobs for !135 with live-initial-packets in 38 minutes and 9 seconds (queued for 3 seconds)
detached
Status Job ID Name Coverage
  Preparation
passed #3109607
gst indent

00:00:16

passed #3109605
manifest

00:00:32

 
  Build
passed #3109625
gstreamer
build cerbero cross-android universal

00:10:21

passed #3109632
gst-ios-13.2
build cerbero cross-ios universal

00:19:33

passed #3109627
gstreamer
build cerbero cross win32

00:06:05

passed #3109629
gstreamer
build cerbero cross win64

00:03:37

passed #3109623
gstreamer
build cerbero fedora x86_64

00:04:28

passed #3109631
gst-macos-10.15
build cerbero macos x86_64

00:09:32

passed #3109614
build clang fedora x86_64

00:04:36

manual #3109620
1809 windows docker allowed to fail manual
build msys2
passed #3109609
build nodebug fedora x86_64

00:03:38

passed #3109610
build static fedora x86_64

00:03:19

passed #3109612
build static nodebug fedora x86_64

00:03:14

passed #3109743
1809 windows docker
build vs2017 amd64

00:17:22

passed #3110721
1809 windows docker
build vs2017 x86

00:14:23

failed #3109616
1809 windows docker
build vs2017 amd64

00:05:01

failed #3109742
1809 windows docker
build vs2017 x86

01:03:31

failed #3109618
1809 windows docker
build vs2017 x86

00:05:00

 
  Test
passed #3109633
gstreamer
check fedora

00:05:37

passed #3109634
gstreamer
integration testsuites fedora 1/4

00:11:59

passed #3109635
gstreamer
integration testsuites fedora 2/4

00:17:35

passed #3109636
gstreamer
integration testsuites fedora 3/4

00:15:35

passed #3109637
gstreamer
integration testsuites fedora 4/4

00:13:29

 
  Integrate
passed #3109638
cross-android universal examples

00:05:15

passed #3109640
gst-ios-13.2
cross-ios universal examples

00:03:30

passed #3109639
documentation

00:05:58