Adds isStandalone to ImageResource so Image can used an UnclippedState.

isStandalone should be true for "data:" URLs and non-inlined images
(preventInlining=true or repeatStyle=Both). In other words, it should
actually rarely be false (IE6/7).

Fixes issue 7403

Change-Id: I832fd71e17e447e642de134e7a556befa1a7f0f1