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/shared/compositing/lib.rs

@@ -148,6 +148,9 @@ pub enum CompositorMsg {
     /// Create a new image key. The result will be returned via the
     /// provided channel sender.
     GenerateImageKey(IpcSender<ImageKey>),
+    /// The same as the above but it will be forwarded to the pipeline instead
+    /// of send via a channel.
+    GenerateImageKeysForPipeline(PipelineId),
     /// Perform a resource update operation.
     UpdateImages(SmallVec<[ImageUpdate; 1]>),
 
@@ -294,12 +297,24 @@ impl CrossProcessCompositorApi {
     }
 
     /// Create a new image key. Blocks until the key is available.
-    pub fn generate_image_key(&self) -> Option<ImageKey> {
+    pub fn generate_image_key_blocking(&self) -> Option<ImageKey> {
         let (sender, receiver) = ipc::channel().unwrap();
         self.0.send(CompositorMsg::GenerateImageKey(sender)).ok()?;
         receiver.recv().ok()
     }
 
+    /// Sends a message to the compositor for creating new image keys.
+    /// The compositor will then send a batch of keys over the constellation to the script_thread
+    /// and the appropriate pipeline.
+    pub fn generate_image_key_async(&self, pipeline_id: PipelineId) {
+        if let Err(e) = self
+            .0
+            .send(CompositorMsg::GenerateImageKeysForPipeline(pipeline_id))
+        {
+            warn!("Could not send image keys to Compositor {}", e);
+        }
+    }
+
     pub fn add_image(
         &self,
         key: ImageKey,