Allow global transactions to nest.

This change fixes a number of small glitches that can occur when
multiple components in the same process are updating surfaces.
One would expect that updates to disjoint sets of surfaces would
not collide but this is not the case.  The first component to
close the global transaction causes all pending updates to
be applied, including those that another component might not
have finished setting up if it also had an open transaction
at the same time.

Change-Id: I99345958581abbe0e1e325a5bcba37e8941a313a