Introduce the videoconvertscale element

Now that videoconvert and videoscale's are both based on
GstVideoConverter and are using the exact same code, it makes much more
sense to have one element doing the two operation, and it can be
more efficient in some cases (one single path for both operations).

This removes the `videoscale` and `videoconvert` plugins but keeps the element
but makes them also do both operations (adding some APIs to each element).

There is a small change in API for the `videoscale:dither` property which
was previously a totally unused boolean, it is now an enum and is used.
28 jobs for !639 with videoconvertscale in 30 seconds (queued for 11 seconds)
latest detached
Status Job ID Name Coverage
  Preparation
passed #2419914
gst indent

00:00:30

manual #2419912
manual
manifest
 
  Build
created #2419931
gstreamer
build cerbero cross-android universal
created #2419940
gst-ios-13.2
build cerbero cross-ios universal
created #2419934
gstreamer
build cerbero cross win32
created #2419936
gstreamer
build cerbero cross win64
created #2419929
gstreamer
build cerbero fedora x86_64
created #2419938
gst-macos-10.15
build cerbero macos x86_64
created #2419921
build clang fedora x86_64
created #2419927
docker windows 1809 allowed to fail manual
build msys2
created #2419915
build nodebug fedora x86_64
created #2419917
build static fedora x86_64
created #2419919
build static nodebug fedora x86_64
created #2419922
docker windows 1809
build vs2017 amd64
created #2419925
docker windows 1809
build vs2017 x86
 
  Test
created #2419941
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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gstreamer
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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gstreamer
valgrind good
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gstreamer
valgrind ugly
 
  Integrate
created #2419951
cross-android universal examples
created #2419953
gst-ios-13.2
cross-ios universal examples
created #2419952
documentation