Fix vertical alignment at the root of an IFC (#31636)

At the root of an inline formatting context, we used its vertical-align
in order to compute the strut. That was wrong, since vertical-align
on a block container shouldn't affect the contents, it should only
affect the alignment of the block container (if it's inline-level)
within the parent IFC.

This was only working well if the block container was block-level, since
effective_vertical_align_for_inline_layout returned `baseline` for
block-level boxes.

Instead of the outer display type, this patch changes the logic to check
whether we are at the root of the IFC.

tests/wpt/meta/css/CSS2/linebox/inline-formatting-context-011.xht.ini

@@ -1,2 +0,0 @@
-[inline-formatting-context-011.xht]
-  expected: FAIL