Cache image properties and frame durations.

	Cache image properties and frame durations.
	Create NSImage and TIFF representations from CGImage, lazily, as needed for
	dragging and element info dictionary.

        Reviewed by John.

        * WebCoreSupport.subproj/WebImageData.h:
        * WebCoreSupport.subproj/WebImageData.m:
        (-[WebImageData dealloc]):
        (-[WebImageData size]):
        (-[WebImageData propertiesAtIndex:]):
        (-[WebImageData _frameDurationAt:]):
        (-[WebImageData _frameDuration]):
        * WebCoreSupport.subproj/WebImageRenderer.h:
        * WebCoreSupport.subproj/WebImageRenderer.m:
        (-[WebImageRenderer dealloc]):
        (-[WebImageRenderer TIFFRepresentation]):
        (-[WebImageRenderer image]):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@8006 268f45cc-cd09-0410-ab3c-d52691b4dbfc