LibWeb+LibImageDecoderClient: Reuse ImageDecoder service process

The overhead from spawning a new ImageDecoder for every decoding job is
way too large and causing other problems as well (#5421)

Let's keep the same decoder open and reuse it as long as it's working.

Userland/Libraries/LibImageDecoderClient/Client.h

@@ -44,7 +44,7 @@ struct DecodedImage {
     Vector<Frame> frames;
 };
 
-class Client
+class Client final
     : public IPC::ServerConnection<ImageDecoderClientEndpoint, ImageDecoderServerEndpoint>
     , public ImageDecoderClientEndpoint {
     C_OBJECT(Client);
@@ -54,9 +54,13 @@ public:
 
     Optional<DecodedImage> decode_image(const ByteBuffer&);
 
+    Function<void()> on_death;
+
 private:
     Client();
 
+    virtual void die() override;
+
     virtual void handle(const Messages::ImageDecoderClient::Dummy&) override;
 };