remember to check for !timing.enabled when building animations
removed MgNode.enabled property, redundant with MgLayer.alpha

Removing it also avoids the thorny problem of how to animate that
boolean value. Layers are still culled from the renderer / layer tree
when alpha = 0, so we don't really lose anything.

One note: if we add CA-style background filters, it's not possible to
cull layers with opacity = 0, if they have certain kinds of background
filter / compositing filter / mask combinations, but we'll cross that
bridge if/when necessary.
added MgGroupLayer.flattensSublayers property

This switches the CALayer subclass to the default MgFlatteningCALayer.
Currently this doesn't attempt to preserve regions of the DOD outside
the layer bounds.