Fix image decoding to separately decode image meta data from actual image bits. I

	Fix image decoding to separately decode image meta data from actual image bits.  I
	incorrectly consolidated decode of meta data and image bits resulting in a huge
	performance regression.

	Double size of WebCore cache on lower end machines.  On the PLT run on machines with
	256MB of memory, too many images were being evicted, causing a re-decode on the PLT.
	Upping the lower limit of the cache size ensure that no images are evicted (this
	goes hand-in-hand with the change to the minimum object size from 32K to 40K).

        Reviewed by Ken.

        * WebCoreSupport.subproj/WebImageData.h:
        * WebCoreSupport.subproj/WebImageData.m:
        (+[WebImageData initialize]):
        (-[WebImageData _commonTermination]):
        (-[WebImageData _invalidateImages]):
        (-[WebImageData _invalidateImageProperties]):
        (-[WebImageData imageAtIndex:]):
        (-[WebImageData propertiesAtIndex:]):
        (-[WebImageData _cacheImages:allImages:]):
        (-[WebImageData decodeData:isComplete:callback:]):
        (-[WebImageData incrementalLoadWithBytes:length:complete:callback:]):
        * WebView.subproj/WebPreferences.m:
        (+[WebPreferences initialize]):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@8257 268f45cc-cd09-0410-ab3c-d52691b4dbfc