This allows applications to request enabling/disabling of
double-buffering of their VCL frame and all its children. It works
after-the-fact, too: so in case the start center creates the frame and
later that frame is reused for Writer, then Writer can turn on
double-buffering, still.

From a user's point of view, this means that next to
VCL_DOUBLEBUFFERING_FORCE_ENABLE, there is now also a
VCL_DOUBLEBUFFERING_ENABLE environment variable that enables a safe
subset that is not supposed to draw directly at all. Enable this for
Writer only, for now.

Change-Id: Ie2cbf7d467eae2cee37fb58a1efc0a8984204408