Reviewed by Dave Hyatt.

        - fix <rdar://problem/6771547> Appending to a RenderText causes all its
          lines to be dirtied

        Test: fast/repaint/text-append-dirty-lines.html

        * rendering/RenderText.cpp:
        (WebCore::RenderText::setTextWithOffset): If no text boxes intersected
        with or came after the new text, dirty the last root box, since the new
        text may begin there. Having at least one dirty line prevents the
        incorrect dirtying of all lines later during layoutInlineChildren().

LayoutTests:

        Reviewed by Dave Hyatt.

        - test for <rdar://problem/6771547> Appending to a RenderText causes all its
          lines to be dirtied

        * fast/repaint/text-append-dirty-lines.html: Added.
        * platform/mac/fast/repaint/text-append-dirty-lines-expected.checksum: Added.
        * platform/mac/fast/repaint/text-append-dirty-lines-expected.png: Added.
        * platform/mac/fast/repaint/text-append-dirty-lines-expected.txt: Added.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@42341 268f45cc-cd09-0410-ab3c-d52691b4dbfc