Reviewed by Eric Seidel.

        Fix for Pattern transformations in Chromium/Skia.  This required
        reworking Pattern a bit to be more like the Gradient implementation.
        In particular, it now holds an m_pattern reference to the
        platform-specific implementation, and passes along changes to the
        m_patternSpaceTransformation, in the same way that Gradient does for
        m_gradientSpaceTransformation.  This is necessary since Skia creates the
        platform-specific pattern (SkShader) once, rather than recreating it
        on each draw.
        For platforms other than Skia, m_pattern is unused, they will
        continue to use the static createPlatformPattern(), and the new
        notification functions are stubbed out.  Other platforms can switch to
        the new implementation if they so choose.

        https://bugs.webkit.org/show_bug.cgi?id=24534

        Covered by svg/custom/pattern-y-offset.svg,
        svg/custom/pattern-cycle-detection.svg, and many more.

        * platform/graphics/Pattern.cpp:
        (WebCore::Pattern::Pattern):
        Initializer for m_pattern.
        (WebCore::Pattern::~Pattern):
        call platformDestroy().
        (WebCore::Pattern::setPatternSpaceTransform):
        Pass along the transform via setPlatformPatternSpaceTransform().
        (WebCore::Pattern::platformDestroy):
        (WebCore::Pattern::setPlatformPatternSpaceTransform):
        Stub implementations for non-skia platforms.
        * platform/graphics/Pattern.h:
        * platform/graphics/skia/GraphicsContextSkia.cpp:
        (WebCore::GraphicsContext::setPlatformFillPattern):
        (WebCore::GraphicsContext::setPlatformStrokePattern):
        Call platformPattern() instead of static version.
        Since Pattern now owns its SkShader, no need to unref here.
        * platform/graphics/skia/PatternSkia.cpp:
        (WebCore::Pattern::platformDestroy):
        Unref the SkShader on destroy.
        (WebCore::Pattern::platformPattern):
        Create the platform pattern (SkShader) once, and cache it.
        (WebCore::Pattern::setPlatformPatternSpaceTransform):
        Set the shader's local matrix from the m_patternSpaceTransformation.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@54203 268f45cc-cd09-0410-ab3c-d52691b4dbfc