qtmux: Use size of first closed caption buffer in prefill mode

It must be accurate for all samples to work in Final Cut properly, so
the best we can do is to assume that all samples are the same as the
first. Bigger samples are truncated, smaller samples are padded.
15 jobs for qtmux-prefill-caption-sizes in 56 minutes and 11 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #332243
gst indent

00:00:16

passed #332242
manifest

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  Build
passed #332244
build fedora x86_64

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passed #332245
build nodebug fedora x86_64

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  Test
passed #332246
check fedora

00:04:53

passed #332247
integration testsuites fedora

00:08:11

 
  Full Builds
passed #332249
build cerbero android universal

00:13:41

passed #332250
build cerbero cross win32

00:09:53

passed #332251
build cerbero cross win64

00:07:25

passed #332248
build cerbero fedora x86_64

00:08:43

 
  Full Tests
passed #332253
valgrind bad

00:11:11

passed #332254
valgrind ges

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passed #332252
valgrind good

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  Apps
passed #332255
android universal examples

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  Deploy
passed #332256
documentation

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