layout: Properly count characters when segmenting IFC text (#42399)

There was a bug where characters were not properly counted when
segmeting IFC text. Immediately incrementing the
`current_character_index` meant that the count was always one character
off. This character count is mainly used for drawing selections and
really matters when multiple text segments are in a single IFC. This
change fixes that by counting characters in the same way we were
counting byte indices for the text.

Testing: This change adds a Servo-specific WPT-style test. As it is
quite
difficult to reproduce the correct display in a different way, a
mismatch test
is used. Since this is mainly about appearance and is very specific to
our
implementation the test is Servo-specific.
Fixes: #42354.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

tests/wpt/mozilla/meta/MANIFEST.json

@@ -136,6 +136,19 @@
   },
   "reftest": {
    "appearance": {
+    "input-text-caret-mixed-language.html": [
+     "9fd52d39a076d9d03876efd93af30402e17e5ee0",
+     [
+      null,
+      [
+       [
+        "/_mozilla/appearance/input-text-caret-mixed-language-ref.html",
+        "!="
+       ]
+      ],
+      {}
+     ]
+    ],
     "input-textual-definite-width.html": [
      "59f1797fab3d1889bbcb7863178c6c5ef9e54f19",
      [
@@ -8269,6 +8282,10 @@
     []
    ],
    "appearance": {
+    "input-text-caret-mixed-language-ref.html": [
+     "ff0b91f2a8857fdefa4a8a3d7d75e06e53e699cf",
+     []
+    ],
     "input-textual-definite-width-ref.html": [
      "7a256be23b3c084bb8477d7380a8b1801ec07f05",
      []