LibWeb: Fix mismatching Resource subclass types

This was a confusing bug: ImageStyleValue loaded its image resource as
a Generic resource, while HTMLImageElement loaded as Image.

This patch fixes the issue and adds an assertion to verify that we only
share resources that have the same C++ client class type.

Libraries/LibWeb/Loader/ImageResource.h

@@ -54,6 +54,9 @@ public:
 protected:
     ImageResource* resource() { return static_cast<ImageResource*>(ResourceClient::resource()); }
     const ImageResource* resource() const { return static_cast<const ImageResource*>(ResourceClient::resource()); }
+
+private:
+    virtual Resource::Type client_type() const override { return Resource::Type::Image; }
 };
 
 }

Libraries/LibWeb/Loader/Resource.cpp

@@ -99,6 +99,8 @@ void ResourceClient::set_resource(Resource* resource)
         m_resource->unregister_client({}, *this);
     m_resource = resource;
     if (m_resource) {
+        ASSERT(resource->type() == client_type());
+
         m_resource->register_client({}, *this);
 
         // Make sure that reused resources also have their load callback fired.

Libraries/LibWeb/Loader/Resource.h

@@ -55,6 +55,8 @@ public:
     static NonnullRefPtr<Resource> create(Badge<ResourceLoader>, Type, const LoadRequest&);
     virtual ~Resource();
 
+    Type type() const { return m_type; }
+
     bool is_loaded() const { return m_loaded; }
 
     bool is_failed() const { return m_failed; }
@@ -95,6 +97,8 @@ public:
     virtual void resource_did_fail() { }
 
 protected:
+    virtual Resource::Type client_type() const { return Resource::Type::Generic; }
+
     Resource* resource() { return m_resource; }
     const Resource* resource() const { return m_resource; }
     void set_resource(Resource*);