Avoid compositing fixed-position elements if they cannot scroll

LayoutTests/compositing/layer-creation/fixed-position-nonscrollable-body-overlap-expected.txt

+Even though we can opt-out of fixed-position compositing for unscrollable fixed-position containers, we still need to composite fixed-position layers that need compositing for other reasons such as overlap.
+
+(GraphicsLayer
+  (bounds 800.00 4024.00)
+  (children 1
+    (GraphicsLayer
+      (bounds 800.00 4024.00)
+      (contentsOpaque 1)
+      (children 2
+        (GraphicsLayer
+          (position 10.00 100.00)
+          (bounds 100.00 100.00)
+          (contentsOpaque 1)
+        )
+        (GraphicsLayer
+          (position 10.00 100.00)
+          (bounds 100.00 100.00)
+          (contentsOpaque 1)
+        )
+      )
+    )
+  )
+)
+