Inconsistent baseline-skip in markups with multi-rows commands

[Bertrand Bordage]

Hi,

In such examples, baseline-skip doesn't behave as expected:

%%
#(set-default-paper-size "a9")
\markuplines {
  \justified-lines { a a a a a a a a a a a a }
  "a" }
%%

I think this would be avoided if multi-rows commands were really creating a
new markup for each line instead of making a single markup.
A fix for this may probably require a lot of downstream changes.

Bertrand.