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/script/Cargo.toml

@@ -95,6 +95,7 @@ net_traits = { workspace = true }
 nom = "7.1.3"
 num-traits = { workspace = true }
 num_cpus = { workspace = true }
+parking_lot = { workspace = true }
 percent-encoding = { workspace = true }
 phf = "0.11"
 pixels = { path = "../pixels" }