videoscale: fix passthrough ENCODED format

Videoscale can passthrough non-sysmem ENCODED buffer just fine. However,
its pad template prevents it from doing that. This commit opens up its
pad template to allow ENCODED format with feature ANY.

However, as ANY feature is a superset of sysmem feature, special care is
given in _transform_caps function to make sure that if the caps uses
sysmem, it won't have ENCODED in it.

Test cases has been updated to check if videotestsrc can use the format
before testing. As videotestsrc doesn't support ENCODED, failure to do
so will make the test fails.
33 jobs for !467 with passthrough-encoded in 4 minutes and 52 seconds (queued for 17 seconds)
detached
Status Job ID Name Coverage
  Build Docker
passed #3645964
alpine amd64 manifest builder docker

00:00:38

passed #3645963
android docker

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cerbero fedora amd64 docker

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fedora amd64 docker

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gst-indent amd64 docker

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  Preparation
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gst indent

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manual #3645967
manual
manifest
 
  Build
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gstreamer
build cerbero cross-android universal
created #3645986
gst-ios-13.2
build cerbero cross-ios universal
created #3645983
gstreamer
build cerbero cross win32
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gstreamer
build cerbero cross win64
created #3645981
gstreamer
build cerbero fedora x86_64
created #3645985
gst-macos-10.15
build cerbero macos x86_64
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build clang fedora x86_64
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1809 windows docker allowed to fail manual
build msys2
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build nodebug fedora x86_64
created #3645972
build static fedora x86_64
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build static nodebug fedora x86_64
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1809 windows docker
build vs2017 amd64
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1809 windows docker
build vs2017 x86
 
  Test
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gstreamer
check fedora
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gstreamer
integration testsuites fedora 1/4
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integration testsuites fedora 2/4
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integration testsuites fedora 3/4
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gstreamer
integration testsuites fedora 4/4
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gstreamer
valgrind bad
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gstreamer
valgrind base
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gstreamer
valgrind ges
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valgrind good
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gstreamer
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  Integrate
created #3645997
cross-android universal examples
created #3645999
gst-ios-13.2
cross-ios universal examples
created #3645998
documentation