Reviewed by Antonio Gomes.

        [Qt] Save and restore shadow state in GraphicsContextQt
        https://bugs.webkit.org/show_bug.cgi?id=44031

        This is needed after r65362.
        Use a new class ContextShadowParameter to track and keep the shadow
        state. We use this rather than using GraphicsContextState to allow
        possible optimizations (right now only to determine the shadow type,
        but in future it might cover things like cached scratch image,
        persistent shader, etc).

        * platform/graphics/qt/GraphicsContextQt.cpp:
        (WebCore::ContextShadowParameter::):
        (WebCore::ContextShadowParameter::ContextShadowParameter):
        (WebCore::ContextShadowParameter::clear):
        (WebCore::GraphicsContextPlatformPrivate::hasShadow):
        (WebCore::GraphicsContextPlatformPrivate::GraphicsContextPlatformPrivate):
        (WebCore::GraphicsContext::savePlatformState):
        (WebCore::GraphicsContext::restorePlatformState):
        (WebCore::GraphicsContext::drawRect):
        (WebCore::GraphicsContext::drawLine):
        (WebCore::GraphicsContext::strokeArc):
        (WebCore::GraphicsContext::drawConvexPolygon):
        (WebCore::GraphicsContext::fillPath):
        (WebCore::GraphicsContext::strokePath):
        (WebCore::GraphicsContext::fillRect):
        (WebCore::GraphicsContext::fillRoundedRect):
        (WebCore::GraphicsContext::setPlatformShadow):
        (WebCore::GraphicsContext::clearPlatformShadow):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@65420 268f45cc-cd09-0410-ab3c-d52691b4dbfc