I observed allocation becoming a bottleneck when building the depsgraph
with scenes with many simple data-blocks. One of the main culprits was
the struct that encodes relations between nodes in the graph.
Instead of allocating each `Relation` with a separate allocation call,
combine them into a `LinearAllocator`. That is must faster because it
allocates in large chunks and just bumps an offset on each allocation.
In a test file with 30 thousand cube objects, I observe a 1.18x
improvement in depsgraph evaluation time, from 1370 to 1163 ms.
The depsgraph isn't completely re-allocated when it's rebuilt, so the
allocator added in this PR has to be cleared manually. In the future,
it can be used for other structs or potentially strings.
Pull Request: https://projects.blender.org/blender/blender/pulls/137303
d1df97bd48