Don't force a flush before starting subbody

Don't force a flush before starting subbody

This ties together {3, 4, 5} and {7, 8} of

    foo \emph{bar baz quux}.
    \-/|\---/|\-/ \-/ \--/|
     1 2  3  4 5   6   7  8

If max_len is hit somewhere around 4, this will end up as

    ..........................foo
      \emph{bar baz quux}.

Previously, it was possible for a line breaking to be something like

    ..........................foo \emph{
      bar baz quux}.

That is bad when the control sequence is something like \ref. This was
foreseen in 5969b2a: Add tkzGetPoint to start_line_controlseqs. Currently,
splitting still happens with math shifts, but this may not be an issue.