videodecoder: Allow outputting non-writable buffer

Some subclasses might hold a ref of outputting buffer (or memory).
For example, if subclass is outputting a buffer which would be used
as a reference picture, it cannot be writable.
But if there's no meta to add to the outputting buffer, it might not need to
be writable.
34 jobs for !843 with vdec-non-writable in 41 seconds (queued for 4 seconds)
latest detached
Status Job ID Name Coverage
  Build Docker
passed #5067121
alpine amd64 manifest builder docker

00:00:33

passed #5067120
android docker

00:00:30

passed #5067119
cerbero fedora amd64 docker

00:00:10

passed #5067118
fedora amd64 docker

00:00:31

passed #5067122
gst-indent amd64 docker

00:00:10

 
  Preparation
passed #5067124
gst indent

00:00:07

manual #5067123
manual
manifest
 
  Build
created #5067133
gstreamer
build cerbero cross-android universal
created #5067137
gst-ios-13.2
build cerbero cross-ios universal
created #5067134
gstreamer
build cerbero cross win32
created #5067135
gstreamer
build cerbero cross win64
created #5067132
gstreamer
build cerbero fedora x86_64
created #5067136
gst-macos-10.15
build cerbero macos x86_64
created #5067138
1809 gstreamer-windows windows docker
build cerbero msvc x86_64
created #5067128
build clang fedora x86_64
created #5067131
1809 windows docker allowed to fail manual
build msys2
created #5067125
build nodebug fedora x86_64
created #5067126
build static fedora x86_64
created #5067127
build static nodebug fedora x86_64
created #5067129
1809 windows docker
build vs2017 amd64
created #5067130
1809 windows docker
build vs2017 x86
 
  Test
created #5067139
gstreamer
check fedora
created #5067140
gstreamer
integration testsuites fedora 1/4
created #5067141
gstreamer
integration testsuites fedora 2/4
created #5067142
gstreamer
integration testsuites fedora 3/4
created #5067143
gstreamer
integration testsuites fedora 4/4
created #5067147
gstreamer
valgrind bad
created #5067144
gstreamer
valgrind base
created #5067148
gstreamer
valgrind ges
created #5067145
gstreamer
valgrind good
created #5067146
gstreamer
valgrind ugly
 
  Integrate
created #5067149
cross-android universal examples
created #5067151
gst-ios-13.2
cross-ios universal examples
created #5067150
documentation