script: Properly root nodes with animating images (#37689)

This change fixes an issue and makes a few more minor improvements to
the `ImageAnimationState`:

1. Image rooting and unrooted now happens in one step from
   `Window::update_animations_post_reflow`.
2. The `node_to_animating_image_map` is now stored as a shared `RwLock`
   so that it doesn't need to be taken and then replaced in the
`ImageAnimationState` during reflow. This should prevent a hypothetical
issue
   where image animations are restarted during empty reflows.
3. General naming and idiomatic Rust usage improvements.

Testing: This doesn't really have any obvious behavioral changes,
because all
reflows currently trigger a restyle. It becomes a serious problem with
#37677
and this change fixes the failing test there.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/shared/layout/Cargo.toml

@@ -29,6 +29,7 @@ libc = { workspace = true }
 malloc_size_of = { workspace = true }
 malloc_size_of_derive = { workspace = true }
 net_traits = { workspace = true }
+parking_lot = { workspace = true }
 pixels = { path = "../../pixels" }
 profile_traits = { workspace = true }
 range = { path = "../../range" }