Merge "Revert "Revert "SurfaceFlinger: add a sysprop for GL backpressure""" into qt-dev am: 3f646b05fc am: c27d3ddd2f am: e3d94279a8
am: 073933cc72

Change-Id: Ia64219d14f92c29b53a7f006c772d8021b158b38