CanvasRenderingContext2D does not expose its backing store resolution
<rdar://problem/10912486> <http://lists.whatwg.org/htdig.cgi/whatwg-whatwg.org/2012-March/035112.html> proposes to expose this using a backingStorePixelRatio attribute.
Created attachment 136544 [details] Add webkitBackingStorePixelRatio as a read-only attribute to CanvasRenderingContext2D
Fixed in <http://trac.webkit.org/r113780>.