Fix cast in AnchorLayout.as to use correct interface

[dubspeed]

positionVertically() and positionHorizintally() should cast the item to IMeasureDisplayObject instead of IFeathersControl, since they do not utilize anything from that interface.

Since IFeathersControl inherits IMeasureDisplayObject this works, however this fix allows custom classes to be layouted via AnchorLayout as well (e.g. ImageSkin) wihtout the full FeathersControl implementation.

[joshtynjala]

It's very likely that I wrote that code before IMeasureDisplayObject existed, so it was an easy thing to miss! Thank you!