Accessing window buffers

I've just got a quick question about how window buffers are handled in the fd.o 
X server. Say I use Composite to request that that a window gets redirected to 
its own buffer. Then I use XRenderCreatePicture to get a RENDER Picture for 
that buffer. Now, if I then call XMoveResizeWindow on the window in question, 
is my Picture still valid? For a cursory look at the xcompmgr source code, the 
answer would appear to be yes, but I'm not sure. 
 
Now, if the picture is still valid, I presume what is happening is that the old 
window buffer is being freed and a new one allocated. Is there any way for the 
composite manager to get ahold of that window buffer before it is freed? 
 
Sincerely, 
    Rayiner Hashem