This is another cleanup in service of trying to get bs-always working. For context, the last time I sent that series to the mailing list, I was explicitly shooting down backing store in
compDestroyWindow, and @keithp rightly called that ugly. Then I got reports of crashes at window destruction in that area, presumably because I'm getting reference counting wrong somewhere along the line.
It turns out that code all looks a lot simpler and more plausibly correct if I just stop using
pWin->backStorage to mean anything. And indeed I think we can remove it entirely. 1/3 in this series removes a stray mention of
backStorage in exposure handling; 2/3 optimizes
ChangeWindowAttributes to only notify the DDX about changes in backing store state. At that point
compCheckBackingStore can just always do what it's told, and then we're not using
backStorage for anything anymore and can delete it.