encodebin: Ensure that a single segment is pushed into encoders

Following the [design document] encodebin needs to handle sources that
output multiple streams, for that purpose and to make it simpler,
we ensure that a single segment is outputted to the encoders by using
an `identity single-segment=true` at the beginning of streams chains.

[design document]: https://gstreamer.freedesktop.org/documentation/additional/design/encoding.html?gi-language=c#rendering-timelines
25 jobs for encodebin_single_segment in 29 minutes and 58 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Preparation
passed #555805
gst indent

00:00:38

passed #555804
manifest

00:00:24

 
  Build
passed #555815
build cerbero android universal

00:12:13

passed #555816
build cerbero cross win32

00:10:02

passed #555817
build cerbero cross win64

00:05:04

passed #555814
build cerbero fedora x86_64

00:08:27

manual #555819
gst-ios-12.2 allowed to fail manual
build cerbero ios universal
passed #555818
gst-macos-10.14
build cerbero macos x86_64

00:07:05

passed #555810
build clang fedora x86_64

00:08:41

passed #555806
build fedora x86_64

00:09:46

passed #555813
docker windows 1607
build msys2

00:12:41

passed #555807
build nodebug fedora x86_64

00:09:39

passed #555808
build static fedora x86_64

00:14:50

passed #555809
build static nodebug fedora x86_64

00:14:09

passed #555811
docker windows 1607
build vs2017 amd64

00:10:08

passed #555812
docker windows 1607
build vs2017 x86

00:11:37

 
  Test
passed #555820
check fedora

00:04:31

failed #555821
integration testsuites fedora

00:19:44

passed #555825
valgrind bad

00:11:46

passed #555822
valgrind base

00:12:10

passed #555826
valgrind ges

00:05:40

passed #555823
valgrind good

00:12:56

passed #555824
valgrind ugly

00:03:06

 
  Integrate
passed #555827
android universal examples

00:11:37

passed #555828
documentation

00:04:21

 
Name Stage Failure
failed
integration testsuites fedora Test
           Total: 1451
Uploading artifacts...
gst-build/build/meson-logs/: found 2 matching files
validate-logs: found 70501 matching files

Uploading artifacts to coordinator... ok
id=555821 responseStatus=201 Created token=F8fnWm2i
Uploading artifacts...
validate-logs/*.xml: found 1 matching files

Uploading artifacts to coordinator... ok
id=555821 responseStatus=201 Created token=F8fnWm2i
ERROR: Job failed: exit code 1