Not always, but skip it if the new length is smaller.
For context, `path_cache` is a `Vec<Vec<Option<Entity>>>`.
# Objective
Previously, when setting a new length to the `path_cache`, we would:
1. Deallocate all existing `Vec<Option<Entity>>`
2. Deallocate the `path_cache`
3. Allocate a new `Vec<Vec<Option<Entity>>>`, where each item is an
empty `Vec`, and would have to be allocated when pushed to.
This is a lot of allocations!
## Solution
Use
[`Vec::resize_with`](https://doc.rust-lang.org/stable/std/vec/struct.Vec.html#method.resize_with).
With this change, what occurs is:
1. We `clear` each `Vec<Option<Entity>>`, keeping the allocation, but
making the memory of each `Vec` re-usable
2. We only append new `Vec` to `path_cache` when it is too small.
* Fixes#11328
### Note on performance
I didn't benchmark it, I just ran a diff on the generated assembly (ran
with `--profile stress-test` and `--native`). I found this PR has 20
less instructions in `apply_animation` (out of 2504).
Though on a purely abstract level, I can deduce this leads to less
allocation.
More information on profiling allocations in rust:
https://nnethercote.github.io/perf-book/heap-allocations.html
## Future work
I think a [jagged vec](https://en.wikipedia.org/wiki/Jagged_array) would
be much more pertinent. Because it allocates everything in a single
contiguous buffer.
This would avoid dancing around allocations, and reduces the overhead of
one `*mut T` and two `usize` per row, also removes indirection,
improving cache efficiency. I think it would both improve code quality
and performance.
78b5f323f8