aggregator: Add a new start-time-selection mode "now"

In case of this GST_AGGREGATOR_START_TIME_SELECTION_NOW mode,
aggregator will select the start time based on pipeline's
running time when the first buffer is arrived. This mode would be
useful for live streaming but should be carefully used as
late initial buffers could be dropped by segment clipping.
34 jobs for !602 with agg-start-now in 58 seconds (queued for 3 seconds)
latest detached
Status Job ID Name Coverage
  Build Docker
passed #4182739
alpine amd64 manifest builder docker

00:00:32

passed #4182738
android docker

00:00:33

passed #4182736
cerbero fedora amd64 docker

00:00:33

passed #4182735
fedora amd64 docker

00:00:33

passed #4182741
gst-indent amd64 docker

00:00:33

 
  Preparation
passed #4182744
gst indent

00:00:23

manual #4182743
manual
manifest
 
  Build
created #4182762
gstreamer
build cerbero cross-android universal
created #4182768
gst-ios-13.2
build cerbero cross-ios universal
created #4182765
gstreamer
build cerbero cross win32
created #4182766
gstreamer
build cerbero cross win64
created #4182760
gstreamer
build cerbero fedora x86_64
created #4182767
gst-macos-10.15
build cerbero macos x86_64
created #4182751
build clang fedora x86_64
created #4182758
1809 windows docker allowed to fail manual
build msys2
created #4182746
build nodebug fedora x86_64
created #4182748
build static fedora x86_64
created #4182749
build static nodebug fedora x86_64
created #4182753
1809 windows docker
build vs2017 amd64
created #4182755
1809 windows docker
build vs2017 x86
 
  Test
created #4182769
gstreamer
check fedora
created #4182770
gstreamer
integration testsuites fedora 1/4
created #4182771
gstreamer
integration testsuites fedora 2/4
created #4182772
gstreamer
integration testsuites fedora 3/4
created #4182773
gstreamer
integration testsuites fedora 4/4
created #4182778
gstreamer
valgrind bad
created #4182775
gstreamer
valgrind base
created #4182774
gstreamer
valgrind core
created #4182779
gstreamer
valgrind ges
created #4182776
gstreamer
valgrind good
created #4182777
gstreamer
valgrind ugly
 
  Integrate
created #4182780
cross-android universal examples
created #4182782
gst-ios-13.2
cross-ios universal examples
created #4182781
documentation