Auto merge of #18212 - mrobinson:position-sticky, r=emilio

Add support for position:sticky

This leverages the position:sticky support in WebRender to bring basic
support for position:sticky in Servo. There are still some issues with
nested sticky flows as well as a few other corner cases. Tests are
imported from WPT and can be removed once we update to the latest
version.

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components/layout/fragment.rs

@@ -2518,8 +2518,9 @@ impl Fragment {
             return true
         }
 
-        // Fixed position blocks always create stacking contexts.
-        if self.style.get_box().position == position::T::fixed {
+        // Fixed position and sticky position always create stacking contexts.
+        if self.style().get_box().position == position::T::fixed ||
+           self.style().get_box().position == position::T::sticky  {
             return true
         }