Auto merge of #25763 - mrobinson:stacking-contexts-v1, r=SimonSapin

Add initial support for stacking contexts to layout_2020

These changes add initial support for stacking contexts, enabling some parts of z-index to work properly.

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- [ ] These changes fix #___ (GitHub issue number if applicable)

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tests/wpt/mozilla/meta-layout-2020/css/css-position-3/position-sticky-stacking-context.html.ini

@@ -0,0 +1,2 @@
+[position-sticky-stacking-context.html]
+  expected: FAIL

tests/wpt/mozilla/meta-layout-2020/css/opacity_stacking_context_a.html.ini

@@ -0,0 +1,2 @@
+[opacity_stacking_context_a.html]
+  expected: FAIL

tests/wpt/mozilla/meta-layout-2020/css/position_relative_painting_order_a.html.ini

@@ -1,2 +0,0 @@
-[position_relative_painting_order_a.html]
-  expected: FAIL

tests/wpt/mozilla/meta-layout-2020/css/position_relative_stacking_context_a.html.ini

@@ -1,2 +0,0 @@
-[position_relative_stacking_context_a.html]
-  expected: FAIL

tests/wpt/mozilla/meta-layout-2020/css/transform_stacking_context_a.html.ini

@@ -0,0 +1,2 @@
+[transform_stacking_context_a.html]
+  expected: FAIL