Generate toCSSFooValue() for CSSImageValue
https://bugs.webkit.org/show_bug.cgi?id=122051

Reviewed by Andreas Kling.

Clean up static_cast<CSSImageValue*> type casts. It will help to
detect bad cast as well as improve code readability.

Removed unnecessary local variables as well.

No new tests, no behavior changes.

* css/CSSCrossfadeValue.cpp:
(WebCore::subimageKnownToBeOpaque):
* css/CSSCursorImageValue.cpp:
(WebCore::CSSCursorImageValue::~CSSCursorImageValue):
(WebCore::CSSCursorImageValue::updateIfSVGCursorIsUsed):
(WebCore::CSSCursorImageValue::cachedImage):
(WebCore::CSSCursorImageValue::isSVGCursor):
* css/CSSImageGeneratorValue.cpp:
(WebCore::CSSImageGeneratorValue::subimageIsPending):
(WebCore::CSSImageGeneratorValue::cachedImageForCSSValue):
* css/CSSImageSetValue.cpp:
(WebCore::CSSImageSetValue::fillImageSet):
* css/CSSImageValue.h:
* css/CSSValue.cpp:
(WebCore::CSSValue::destroy):
* css/StyleResolver.cpp:
(WebCore::StyleResolver::applyProperty):
(WebCore::StyleResolver::styleImage):
* page/PageSerializer.cpp:
(WebCore::PageSerializer::retrieveResourcesForProperties):
* rendering/style/StylePendingImage.h:
(WebCore::StylePendingImage::data):
(WebCore::StylePendingImage::cssImageValue):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@156601 268f45cc-cd09-0410-ab3c-d52691b4dbfc