lilypond-book-preamble and cropping

[Urs Liska]

I'm not sure if this is actually a *development* request or if it could also be 
solved at the "user" level.

As Werner showed in https://github.com/jperon/lyluatex/issues/268 
(https://github.com/jperon/lyluatex/issues/268) there is an issue with the 
output of scores compiled with `lilypond-book-preamble.ly`. AFAICT this is 
something inherent in the lilypond-book handling which should also be visible 
in any lilypond-book scores.

When compiling with lilypond-book-preamble the score gets sliced in systems, 
and each system is *additionally cropped*. I have never understood the commands 
in that file so I don't know in what order these things happen, I don't even 
know if that lack of the concept of a "page" (we only have a sequence of 
systems now) affects the actual layout process.

Of course you will often want to have the cropped systems, essentially when 
including single systems in a text document, or at the top or the bottom of 
pages. However, when stacking the resulting systems this means that the space 
between systems is inconsistent and generally too narrow. This can sometimes be 
alleviated by adding space between the systems, but sometimes this doesn't 
help. As Werner's example images in the issue report linked above show: when a 
system has much additional stuff above or below (e.g. marks, text or just 
legder lines) this significantly disturbes the vertical spacing.

What I would need to achieve - either by providing some modification of 
lilypond-book-preamble or as a feature request to be added to LilyPond - is a 
compilation mode that behaves like lilypond-book-preamble *without the vertical 
cropping* (but with horizontal cropping). Alternatively (maybe even better) 
would be writing an additional aux file stating the amount of space cropped, at 
least at the top and bottom but maybe for all four sides. Or the original image 
size before cropping. Anything that can be used to infer the space one should 
add before and after the system to rebuild LilyPond's original page layout.

Any ideas?
Thanks
Urs