Reviewed by Eric Carlson.

        With hardware acceleration turned off, video frames never display if poster
        image is specified
        https://bugs.webkit.org/show_bug.cgi?id=34965

        HTMLVideoElement's m_shouldDisplayPosterImage was never updated after
        the movie supplied its first video frame, so the poster would continue to show.

        Fixed by calling updatePosterImage() from mediaPlayerRepaint(), which is called
        each time a new frame is available. updatePosterImage() is cheap.

        Also made updatePosterImage() virtual on HTMLMediaElement to avoid a number of
        ugly casts.

        Test: manual-tests/media-elements/video-replaces-poster.html

        * html/HTMLMediaElement.cpp:
        (WebCore::HTMLMediaElement::noneSupported): Call updatePosterImage() without video check.
        (WebCore::HTMLMediaElement::setNetworkState): Ditto.
        (WebCore::HTMLMediaElement::setReadyState): Ditto.
        (WebCore::HTMLMediaElement::mediaPlayerRepaint): Call udpatePosterImage().
        * html/HTMLMediaElement.h:
        (WebCore::HTMLMediaElement::updatePosterImage): Make this an empty virtual
        method on the base class.
        * html/HTMLVideoElement.h: Override updatePosterImage().

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@54828 268f45cc-cd09-0410-ab3c-d52691b4dbfc