Otherwise it's impossible to get the COW without a white
flash on the screen, because it's on top, mapped immediately,
and unaffected by composite redirection. This makes
initial login ugly when it doesn't need to be.
The composite overlay window code had several misunderstandings of the
workings of the X server, in particular error handling paths would often
double-free objects. Clean all of this up by using resource destruction as
the sole mechanism for freeing resource-based objects.
