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 350-video-convert-fix-number-of-allocated-lines in 22 minutes and 42 seconds (queued for 1 second)
Status Job ID Name Coverage
  Preparation
passed #78706
gst indent

00:00:30

passed #78705
manifest

00:00:19

 
  Build
passed #78707
build fedora x86_64

00:05:38

 
  Test
passed #78708
check fedora

00:03:39

 
  Full Builds
passed #78709
build android arm64 api28

00:03:48

passed #78711
build cerbero cross win32

00:07:14

passed #78712
build cerbero cross win64

00:12:10

passed #78710
build cerbero fedora x86_64

00:12:53