Update snapshot upon saveLayer deferral

bug:8409891

In order to defer fbo-targetting saveLayer operations, it's necessary
to update the snapshot at defer time so that deferred display state
(namely, clip and transform) are fbo relative.

Re-enables deferring, as the issues with saveLayer are fixed.

Change-Id: I74b0779bc732675c747208f0757c3ea85f6dfbed