* Sort stacking contexts and stacking containers by painting order
* fix stealing of stacking containers; fix interleaving with fragments
* actually positioned stacking containers should be stolen too
* update expectations and clean up panic changes
* rework naming and docs
* rename s_c_a_p_s_c to real_s_c_a_p_s_c; fix docs
* rename InlineStackingContainer to AtomicInlineStackingContainer
* rework debug logging to use PrintTree
* clean up docs and PrintTree output
* don't panic unless cfg!(debug_assertions) is true
* update expectations
