Re: Uses only unpure-pure containers to articulate unpure-pure relationships. (issue 7377046)

[mtsolo]

Some code from my patch for issue 3161 is copied over to this patch.
The fact that Grob::pure_height returns Interval (0,0) as default causes
point stencils to be placed in the skyline, potentially effecting
vertical spacing.

Separation_item::boxes uses this (0,0) extent to add extra spacing
height to certain grobs.  To allow this to still happen, we give the
grobs without pure height a small empty extent so that the (almost) full
compliment of extra spacing height is taken into account.  This is why
small-empty-interval exists in lily-library.scm.

https://codereview.appspot.com/7377046/