Bug 778307. Don't shadow mFormat in DrawTargetCG. r=bgirard

This was causing problems with Azure content.

gfx/2d/DrawTargetCG.h

@@ -163,8 +163,6 @@ private:
    */
   void *mData;
 
-  SurfaceFormat mFormat;
-
   RefPtr<SourceSurfaceCGContext> mSnapshot;
 };