Create new image cache per document (#36832)

Rather than sharing the full image cache in a script_thread, the image
cache is now unique per document. This ensures that CSP factors no
longer affect whether the image is retrieved from the cache incorrectly.

To do so, the thread_pool is shared across all caches, but the store is
fresh. Except for the place_holder{image,url}, which are cloned. That's
because the `rippy_data` is only available in the constellation and no
longer accessible at the point that we need to create the document in
the script_thread.

Contrary to the description in #36505, the script_thread still has an
image_cache for this reason: so it has access to the store and
thread_pool to clone it.

With these changes, the two CSP tests no longer flake. Confirmed with
running the following commmand:

```
./mach test-wpt tests/wpt/tests/content-security-policy/generic/ --rerun=10
```

Fixes #36505

Signed-off-by: Tim van der Lippe <tvanderlippe@gmail.com>

components/layout/layout_impl.rs

@@ -452,6 +452,8 @@ impl Layout for LayoutThread {
                 .map(|tree| tree.conditional_size_of(ops))
                 .unwrap_or_default(),
         });
+
+        reports.push(self.image_cache.memory_report(formatted_url, ops));
     }
 
     fn set_quirks_mode(&mut self, quirks_mode: QuirksMode) {