Line breaking, simple-spacer, etc.

[Boris Shingarov]

I am experimenting with some modifications to the line breaking code, and I am 
stuck trying to understand how some of it works.  So far my understanding is 
that Simple_spacer operates on a vector of Grobs, and it is a well-known 
Constrained-QP problem (rods = constraints, springs = quadratic function to 
minimize).  What I don't understand is, if the spacer operates at the level of 
Grobs, which are built at an earlier stage in the pipeline, how are the changes 
necessitated by differences in line breaking, taken into account?  in other 
words, if I take the last measure of a line and place it on the next line, it 
is not just a matter of literally moving that graphic to where the start of the 
next line is, but I also need to draw a clef, key signature, and possibly other 
fundamental things -- but at that stage in the rendering pipeline, is it not 
too late??