layout: Take relative position offsets for inlines and inline-blocks

into account only once.

There were two bugs here: (1) relative position applied to
scanned/unscanned text fragments independently of the container element
that applied that relative position, causing double-counting; (2)
relative position applied to inline block fragments independently of the
wrapped block itself, causing double-counting.

This commit also removes the `cascade_anonymous` function and the
related `Fragment` constructor. They were unused, and their
functionality has been replaced by the `modify_style_for_*` series of
functions.

Closes #7067.

tests/wpt/metadata-css/css21_dev/html4/floats-153.htm.ini

@@ -1,3 +0,0 @@
-[floats-153.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/margin-collapse-001.htm.ini

@@ -1,3 +0,0 @@
-[margin-collapse-001.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/position-relative-032.htm.ini

@@ -1,3 +0,0 @@
-[position-relative-032.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/run-in-relpos-between-001.htm.ini

@@ -1,3 +0,0 @@
-[run-in-relpos-between-001.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/run-in-relpos-between-002.htm.ini

@@ -1,3 +0,0 @@
-[run-in-relpos-between-002.htm]
-  type: reftest
-  expected: FAIL