Reviewed by Dirk Schulze.

        [GTK] Support pixel tests
        https://bugs.webkit.org/show_bug.cgi?id=31518

        Finish support for pixel tests on GTK+.

        * DumpRenderTree/PixelDumpSupport.cpp: Remove unecessary RetainPtr include (only
        works on CoreFoundation systems) and add missing cstdio include.
        * DumpRenderTree/cairo/PixelDumpSupportCairo.cpp: Switch to using the MD5 support found
        in JSC library. MD5.cpp and MD5.h are only available for the Windows build.
        (computeMD5HashStringForBitmapContext): The number of bytes per row should be the row stride
        of the image, not the row stride multiplied by the width. Use JSC MD5 implementation to calculate
        the MD5 hash here. According to glibc manpages, using snprintf to build a string in unsupported
        by the ANSI standard and this fails on Linux, so unroll the loop here.
        * DumpRenderTree/cairo/PixelDumpSupportCairo.h: Correct some constructor misbehavior.
        (BitmapContext::BitmapContext): There is no reason to check the value of the
        m_context member in the constructor and it certainly should not be freed. Instead
        just initialize it with the incoming value.
        * DumpRenderTree/gtk/DumpRenderTree.cpp: Fix the order of includes.
        (dump): Actually call dumpWebViewAsPixelsAndCompareWithExpected when it is time to
        dump pixels.
        * DumpRenderTree/gtk/PixelDumpSupportGtk.cpp: Added.
        (createBitmapContextFromWebView):
        * GNUmakefile.am: Add new files to the build.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@63130 268f45cc-cd09-0410-ab3c-d52691b4dbfc