This is a workaround for a race condition when resizing windows while
there are in-flight glXCopySubBuffer blits happening.
The problem stems from the fact that rectangles for the blits are
described relative to the bottom left of the window and because we can't
guarantee control over the X window gravity used when resizing so the
gravity is typically NorthWest not SouthWest.
This means if you grow a window vertically the server will make sure to
place the old contents of the window at the top-left/north-west of your
new larger window, but that may happen asynchronous to GLX preparing to
do a blit specified relative to the bottom-left/south-west of the window
(based on the old smaller window geometry).
When the GLX issued blit finally happens relative to the new bottom of
your window, the destination will have shifted relative to the top-left
where all the pixels you care about are so it will result in a nasty
artefact making resizing look very ugly!
We can't currently fix this completely, in-part because the window
manager tends to trample any gravity we might set. This workaround
instead simply disables blits for a while if we are notified of any
resizes happening so if the user is resizing a window via the window
manager then they may see an artefact for one frame but then we will
fallback to redrawing the full stage until the cooling off period is
over.
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