wayland backend: do not watch wayland fd for writable
Wayland fd is always writable and will busy-loop. The dispatch function gets called with 0-mask when we need to flush display anyway, so this saves CPU at no visible impact.
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The migration is almost done, at least the rest should happen in the background. There are still a few technical difference between the old cluster and the new ones, and they are summarized in this issue. Please pay attention to the TL:DR at the end of the comment.
Wayland fd is always writable and will busy-loop. The dispatch function gets called with 0-mask when we need to flush display anyway, so this saves CPU at no visible impact.