backend/wayland: Use calloop executor to spawn future

[ids1024]

This should be fixed now that we're using calloop 0.14.4.

Spawning a thread for futures is excessive if we only use it to wait for buffer release.

This seems to be good, but testing this I'm looking into some inconsistent issues that seem to happen with or without this (though it's hard to be sure if there's a change).

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