Issue 4160050 : First attempt at tackling issue 1504

[Mike Solomon]

http://codereview.appspot.com/4160050

I think I have the slopes right, but I have not tocuhed the Y offset yet 
because I don't get the difference between:

local_slope * (segments[i].horizontal_[LEFT] - span.linear_combination 
(feather_dir))

and

pos.linear_combination (feather_dir)

in beam.cc .  Or rather I get that they are different, but I have trouble 
understanding what function they serve.  The second one is more comprehensible 
to me because it's based on the quanted positions, but I have trouble 
understanding what the first one does.  Even if I did understand why it was 
there, I'd likely still be lost - the math will take me more time to slog 
through than I currently have.

Anyway, some help would be appreciated.  If someone feels like tag-teaming &/or 
adopting my patch, please do - I'm performing a piece on April 20th that could 
greatly benefit from this patch & the issue 37 one.  I'd have time to spend on 
making GNU LilyPond t-shirts for my performers to wear if I can get this thing 
nailed by then...

http://codereview.appspot.com/4160050

Cheers,
MS

P.S. Sorry for the pretty prints in the source : I've been using them to 
understand what's going on.  I'll remove them in later drafts.

feather.ly
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