video-converter: fix number of allocated lines

We make an allocator for temporary lines and then use this for all
the steps in the conversion that can do in-place processing.

Keep track of the number of lines each step needs and use this to
allocate the right number of lines.

Previously we would not always allocate enough lines and we would
end up with conversion errors as lines would be reused prematurely.

Fixes #350
8 jobs for master in 15 minutes and 49 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Preparation
passed #78780
gst indent

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passed #78779
manifest

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  Build
passed #78781
build fedora x86_64

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  Test
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check fedora

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  Full Builds
passed #78783
build android arm64 api28

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passed #78785
build cerbero cross win32

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passed #78786
build cerbero cross win64

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passed #78784
build cerbero fedora x86_64

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