* 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
Most tests were only being run for layout-2013, not for layout-2020.
This wasn't great since layout-2020 is now the default.
So this patch unifies the lists of included tests for both layouts.
For layout-2013 this implies adding css/css-content/, css/css-logical/
and css/css-masking/clip/.
For layout-2020 this implies adding several additional css tests, and
also tests like dom/, js/, html/, etc.