Add a basic caching mechanism for ImageKeys. (#37369)

This creates a new method in shared/compositing/lib to generate image
keys that are send over the webview. This does not immediately return
the keys but goes over the constellation to receive the keys from the
IOCompositor. To make this more efficient, we now cache the keys in
image_cache in a simple FIFO order. The old blocking method stays intact
for now but got renamed to make the blocking clear.
The blocking calls that are left are in:
- `components/canvas/canvas_data.rs`
- `components/script/dom/htmlmediaelement.rs`

Testing: WPT tests should cover this as this doesn't change any
functionality.
Fixes: Was mentioned in
https://github.com/servo/servo/issues/37161#issuecomment-2915750051 and
part of https://github.com/servo/servo/issues/37086

---------

Signed-off-by: Narfinger <Narfinger@users.noreply.github.com>
Signed-off-by: gterzian <2792687+gterzian@users.noreply.github.com>
Co-authored-by: gterzian <2792687+gterzian@users.noreply.github.com>

components/script/messaging.rs

@@ -92,6 +92,7 @@ impl MixedMessage {
                 ScriptThreadMessage::SetWebGPUPort(..) => None,
                 ScriptThreadMessage::SetScrollStates(id, ..) => Some(*id),
                 ScriptThreadMessage::EvaluateJavaScript(id, _, _) => Some(*id),
+                ScriptThreadMessage::SendImageKeysBatch(..) => None,
             },
             MixedMessage::FromScript(inner_msg) => match inner_msg {
                 MainThreadScriptMsg::Common(CommonScriptMsg::Task(_, _, pipeline_id, _)) => {