Page Spacer gets confused by next-space leading to unusable layout

[Boris Shingarov]

When typesetting a large volume of markuplines, the Page Spacer gets
confused by the setting of next-space (1.0 by default).  It does
take it into account when calculating the forces, but ignores it when
actually placing the lines on the page.  This leads to horrible layout.
For example, if line height is 3, and printable height is 120, then
the force calculation adds next-space to spring_len, resulting in
only 120/4=30 lines.  But the actual spacing, when the page is produced,
is 3, not 3+1=4.  In other words, at the pitch the lines are spaced,
there is space for 40 lines, resulting in 30 printed and 10 empty lines.

As a workaround, I make the default next-space to be 0.0.  It would be
better to only do it if the line is a markup line, but it is unclear to
me how to get back from the Line_details to the System_spec which
produced it.

Similarly, nonzero padding in "between-title-spacing" produces the
same bad look.  I have to set it to 0.0 in the score:

\paper {
 #(set-paper-size "a6")
 between-title-spacing = #'((padding . 0))
}