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>
8a837778d9