matroskamux: Set DateUTC based on datetime tag

Use timestamp found in a datetime tag for DateUTC, or continue to use
g_get_current_time() in case no tag event was received.

Fixes gstreamer/gst-plugins-good#481
19 jobs for matroskadateutc in 51 minutes and 45 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Preparation
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gst indent

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manifest

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  Build
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build cerbero android universal

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build cerbero cross win32

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build cerbero cross win64

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build cerbero fedora x86_64

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manual #510211
gst-ios-12.2 allowed to fail manual
build cerbero ios universal
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gst-macos-10.14
build cerbero macos x86_64

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build fedora x86_64

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build nodebug fedora x86_64

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build static fedora x86_64

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passed #510205
build static nodebug fedora x86_64

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  Test
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check fedora

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integration testsuites fedora

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valgrind bad

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valgrind ges

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valgrind good

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  Integrate
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android universal examples

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documentation

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