Bug 282727

When descending through a layer that has an animated transform, the layerExtent is computed using the post-transform animation extents and the transform itself isn't pushed onto the geometry map. That means for all descendants, overlap bounds are no longer 'absolute', since they aren't transformed. This generally works ok for overlap testing, since everything within the transform stacking context is in the same coordinate space. updateAllowsBackingStoreDetaching compares these untransformed coordinates against the layout viewport though, and this is invalid, leading to backing stores not being detached.