fix(math): Avoid page breaks before display math equations

[Omikhleia]

See #2160.
This is a conservative workaround.
Introducing the predisplay/postdisplay penalties is the right thing to do, but a more general solution to the full issue requires a more subtle handling of in-paragraph display math equations, which is not considered here.

[alerque]

v0.15.6 is a bit delayed, I'm chasing an unexpected panic in some builds and some build concerns. At first blush this seems to make sense to me but I don't have time to dig into alternatives. If you think this should go out in the current batch of math fixes and features please feel free to target the milestone and I'll review before releasing.

[Omikhleia]

If you think this should go out in the current batch of math fixes and features please feel free to target the milestone and I'll review before releasing.

Yes, it's still better than nothing here -- so adding the 0.15.6 milestone if it's still doable.
I commented the original issue for the "eventual" right way to fix this properly at some point, but I might never reach that point any time soon, while at least here we can have something fairly decent working with the current implementation, without taking too much risks.
More generally, I'm nearly done with what I wanted in the "first" batch of mathematical "fixes & improvements":

• I see you moved Math font size does not follow the current normal text size #2145 to 0.15.7 and I agree we need to better address it (for now, I can go for the workaround in my math booklet in progress, and re-consider this point later)
• I'm working on the "operator table" from MathML, but it's a "big" thing to get right... It's on tracks, but I can't say when I'll complete it (this week? next one? later? doh!)
• And the next topic would be a proper handling of (at least top) accents and (horizontal) stretchy operators, not a big thing but it's highly depending on the previous point.

So I'm still adding stuff to it, but feel safe to "cut the release" at your convenience :)