Skip alloc when updating animation path cache (#11330)

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.

crates/bevy_animation/src/lib.rs

@@ -710,7 +710,9 @@ fn apply_animation(
     );
 
     if animation.path_cache.len() != animation_clip.paths.len() {
-        animation.path_cache = vec![Vec::new(); animation_clip.paths.len()];
+        let new_len = animation_clip.paths.len();
+        animation.path_cache.iter_mut().for_each(|v| v.clear());
+        animation.path_cache.resize_with(new_len, Vec::new);
     }
     if !verify_no_ancestor_player(maybe_parent, parents) {
         warn!("Animation player on {:?} has a conflicting animation player on an ancestor. Cannot safely animate.", root);