compositing: Send entire scene's scroll offsets when sending WebRender display lists (#31892)

WebRender does not preserve spatial tree offsets when updating the
spatial tree. Updating the spatial tree of a pipeline can also
update the spatial tree of child pipelines. This change ensures that
WebRender always gets the scroll offsets of the entire scene when
modifying display lists in a way that may rebuild the spatial tree.

Fixes #31807.

tests/wpt/meta/MANIFEST.json

@@ -261166,6 +261166,19 @@
        {}
       ]
      ],
+     "transform-iframe-scroll-position.html": [
+      "efb7bab532606cd9893a4cb4223dbf4b7baa6f1d",
+      [
+       null,
+       [
+        [
+         "/css/css-transforms/transform-iframe-scroll-position-ref.html",
+         "=="
+        ]
+       ],
+       {}
+      ]
+     ],
      "transform-image-001.html": [
       "0565b8dbeeb86b82993847a139c8f38b66c0b163",
       [
@@ -416071,6 +416084,10 @@
        "84f079c90bcb590e81ba39753edf723bcb123858",
        []
       ],
+      "transform-iframe-scroll-position-contents.html": [
+       "8efcdafc83cde63f89d56ea437a4852dd82cc206",
+       []
+      ],
       "transform-lime-square.png": [
        "8f939993332e1101b921615723ec6067f3bb90a3",
        []
@@ -416208,6 +416225,10 @@
       "b674c88d82f8a806a8a1cd20040302766d825202",
       []
      ],
+     "transform-iframe-scroll-position-ref.html": [
+      "e4d5da75d7a762b6c346640b2c72339a52d350ab",
+      []
+     ],
      "transform-image-ref.html": [
       "301c0f94bb7806caad2444583f3642d49aa4c969",
       []