misplaced-note-head bug (issue 5303)

[Lukas-Fabian Moser]

Folks,

I think I isolated the rounding (?) issue leading to the misplaced note 
head in https://sourceforge.net/p/testlilyissues/issues/5303/

Please forgive the horrible C/C++ jumble - it's been quite long since I 
did this kind of stuff and usually only ever wrote vanilla C:

#include <stdio.h>

double a = -0x1.7ffffffffffffp+1;

int main(void) {
  printf("%a, as float: %f, as int: %d\n", a, a, int (a));
}

Compiled with gcc (4.8.5, 5.5.0, 6.5.0, 7.4.0 on my x86_64) this displays:

-0x1.7ffffffffffffp+1, as float: -3.000000, as int: -2

I'm not familiar with technicalities involved and thought I'd ask here 
because I'm sure somebody here (David K.?) will be able to explain 
what's happening here much faster than I could ever hope by reading up 
on float representations, rounding issues, etc.

Best
Lukas