0.18 backport: seat: Don't forget to destroy touch points on touch up
This is a cherry-pick of !4973 (merged) as otherwise touch drag and drop can't work on 0.18.
(cherry picked from commit fef4f363)
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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.
This is a cherry-pick of !4973 (merged) as otherwise touch drag and drop can't work on 0.18.
(cherry picked from commit fef4f363)