In the tiled compositor ensure_delta() can be called from multiple threads,
but without any threading synchronization. This worked fine when the node
only supported absolute transform: multiple threads would do the same work
and assign delta to the same values.
With the addition of relative transform in #115947 a code which adjusts
previously calculated delta was added, leading to possible double-applying
relative transform.
The solution is to avoid multiple threads modifying the same data by using
a double-locked check.
This issue does not happen in 4.2 (main branch) because it switched to full
frame compositor, which works differently.
Sergey Sharybin