avformat/matroskaenc: write DisplayWidth and DisplayHeight elements only if they differ from PixelWidth and PixelHeight
Signed-off-by: James Almer <jamrial@gmail.com>

The spec says:
"Mandatory elements with a default value may be left out of the file. In the absence
of a mandatory element, the element's default value is used."
Reviewed-by: Hendrik Leppkes <h.leppkes@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>

Implements part of ticket #4347
Tested-by: Dave Rice <dave@dericed.com>
Tested-by: Jerome Martinez <jerome@mediaarea.net>
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: James Almer <jamrial@gmail.com>

The durations are never written in that situation.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: James Almer <jamrial@gmail.com>

Previously, we used a different list of checks when deciding whether to
write a set of tags at all than we did when deciding whether to write an
individual tag in the set. This resulted in sometimes writing an empty
tag master and seekhead. Now we use mkv_check_tag_name everywhere, so
if a dictionary is entirely composed of tags we skip, we don't write a
tag master at all.

This affected the test file, since "language" was on one list but not
the other, so we were writing an empty tag master there. The test hash
is updated to reflect that change.

This tests automatic insertion of the vp9_superframe BSF as well as
ensuring that the colorspace properties in the video header can be
modified when remuxing (-c:v copy).