Very rarely we can see a deadlock between import_commit and destroy_buffer where:
- destroy_buffer is waiting on `wlr_buffer_map_lock` and has `wlserver_lock` (from up the chain)
- import_commit has `wlr_buffer_map_lock` and is waiting on `wlserver_lock`
To avoid this, we simply replace the lock_guard in import_commit with a unique_lock and manually unlock this before going into the section where we need to lock the wl_server.
This is safe for a few reasons:
- 1: All accesses to wlr_buffer_map are done before this lock.
- 2: destroy_buffer cannot be called from this buffer before now
as it only happens because of the signal added below.
- 3: "References to elements in the unordered_map container remain
valid in all cases, even after a rehash."
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