Add buffer attach while waiting for inistial subsurface frame callback

[maxhbooth]

Weirdly, this only seems to affect directly running xwayland-xdg-shell, and not wprsd/wprsc. Which seems bad because that might mean the attach is leaking before the frame callback, and that the position isn't getting initialized properly?

[maxhbooth]

abandoning for https://github.com/wayland-transpositor/wprs/pull/26