Do not merge text fragments of differing colors

The display list item for a line of text has a single color assigned for
it, so text fragments with different colors cannot be merged.

There is no issue number for this, as far as I know. I found this while
trying an internal program that uses red asterisks for required text
fields.

tests/wpt/metadata-css/css21_dev/html4/content-inherit-002.htm.ini

@@ -1,3 +0,0 @@
-[content-inherit-002.htm]
-  type: reftest
-  expected: FAIL