Calculate stat_align() per panel

[teunbrand]

This PR aims to fix #5227 and fix #5176, thereby superseding #5177.

Briefly, to prevent the small offset adjust taking on too large values, we calculate the unique x-locations ± offset per panel instead of globally. I think it comes with the following trade-offs.

• - It has to be computed for every panel, possibly being less efficient than doing it once globally.
• + To compensate, I tried making determining the unique x-positions faster, by
  • Not taking into account 0-crossings when there is only one group and no stacking needs to occur
  • Unlooping the groupwise 0-crossings calculation for efficiency

[thomasp85]

Can you add a unit test that covers what this fixes?