Vertical spacing, distinguish stretchability/compressibility (issue4517136)

[joeneeman]

http://codereview.appspot.com/4517136/diff/1001/lily/align-interface.cc
File lily/align-interface.cc (left):

http://codereview.appspot.com/4517136/diff/1001/lily/align-interface.cc#oldcode230
lily/align-interface.cc:230: dy = max (dy,
Page_layout_problem::get_fixed_spacing (elems[j-1], elems[j],
spaceable_count, pure, start, end));
I think this is still necessary because of alignment-distances.

http://codereview.appspot.com/4517136/diff/1001/lily/simple-spacer.cc
File lily/simple-spacer.cc (right):

http://codereview.appspot.com/4517136/diff/1001/lily/simple-spacer.cc#newcode230
lily/simple-spacer.cc:230: for (++i; i < sorted_springs.size (); i++)
It seems like this loop will never see i=0, even if it is active.

http://codereview.appspot.com/4517136/diff/1001/lily/spring.cc
File lily/spring.cc (left):

http://codereview.appspot.com/4517136/diff/1001/lily/spring.cc#oldcode55
lily/spring.cc:55: // -infinity_f works fine for now.
So now, you make fixed springs have a blocking_force_ of zero instead of
-infinity_f. Does that work properly in simple-spacer? It seems to me
like it will sort springs in the wrong order (and also line 237 won't
work as intended).

http://codereview.appspot.com/4517136/diff/1001/lily/spring.cc
File lily/spring.cc (right):

http://codereview.appspot.com/4517136/diff/1001/lily/spring.cc#newcode51
lily/spring.cc:51: // Simple_spacer::compress_line() depends on the
contition above.
condition

http://codereview.appspot.com/4517136/