Introduce wlr_format_set
This type adds a container for formats + modifiers.
This is cherry-picked from #1355. wlr_format_set is only used in one place for now, but it'll be useful in other places too in #1641.
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This type adds a container for formats + modifiers.
This is cherry-picked from #1355. wlr_format_set is only used in one place for now, but it'll be useful in other places too in #1641.