Fixed <rdar://problem/3905789> Burn Disc image vibrates rapidly

	Fixed <rdar://problem/3905789> Burn Disc image vibrates rapidly

	Restrict our support for animated images to GIF.  We used to
	use presence of more than one image in a resource to determine
	if an image should be animated.  This caused us to animate icns!
	If we ever support any other animated image formats we'll have
	to extend.

        Reviewed by Hyatt.

        * WebCoreSupport.subproj/WebImageData.h:
        * WebCoreSupport.subproj/WebImageData.m:
        (-[WebImageData shouldAnimate]):
        * WebCoreSupport.subproj/WebImageRenderer.m:
        (-[WebImageRenderer _startOrContinueAnimationIfNecessary]):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@8178 268f45cc-cd09-0410-ab3c-d52691b4dbfc