LibWeb: Add basic implementation of has_a_rendering_opportunity()

Return true only if we are ready to repaint. This fixes the issue where
requestAnimationFrame() was invoked more than once between repaints.

Userland/Libraries/LibWeb/HTML/Navigable.cpp

@@ -2073,8 +2073,11 @@ bool Navigable::has_a_rendering_opportunity() const
     // or whether the document's visibility state is "visible".
     // Rendering opportunities typically occur at regular intervals.
 
-    // FIXME: We should at the very least say `false` here if we're an inactive browser tab.
+    // FIXME: Return `false` here if we're an inactive browser tab.
-    return true;
+    auto browsing_context = const_cast<Navigable*>(this)->active_browsing_context();
+    if (!browsing_context)
+        return false;
+    return browsing_context->page().client().is_ready_to_paint();
 }
 
 // https://html.spec.whatwg.org/multipage/nav-history-apis.html#inform-the-navigation-api-about-aborting-navigation

Userland/Libraries/LibWeb/Page/Page.h

@@ -307,6 +307,7 @@ public:
     virtual void inspector_did_execute_console_script([[maybe_unused]] String const& script) { }
 
     virtual void schedule_repaint() = 0;
+    virtual bool is_ready_to_paint() const = 0;
 
 protected:
     virtual ~PageClient() = default;

Userland/Libraries/LibWeb/SVG/SVGDecodedImageData.cpp

@@ -49,6 +49,7 @@ public:
     virtual void request_file(FileRequest) override { }
     virtual void paint(DevicePixelRect const&, Gfx::Bitmap&, Web::PaintOptions = {}) override { }
     virtual void schedule_repaint() override { }
+    virtual bool is_ready_to_paint() const override { return true; }
 
 private:
     explicit SVGPageClient(Page& host_page)

Userland/Services/WebContent/PageClient.cpp

@@ -96,6 +96,11 @@ void PageClient::schedule_repaint()
         m_repaint_timer->start();
 }
 
+bool PageClient::is_ready_to_paint() const
+{
+    return m_paint_state == PaintState::Ready;
+}
+
 void PageClient::ready_to_paint()
 {
     auto old_paint_state = exchange(m_paint_state, PaintState::Ready);

Userland/Services/WebContent/PageClient.h

@@ -30,6 +30,7 @@ public:
     static void set_use_gpu_painter();
 
     virtual void schedule_repaint() override;
+    virtual bool is_ready_to_paint() const override;
 
     virtual Web::Page& page() override { return *m_page; }
     virtual Web::Page const& page() const override { return *m_page; }

Userland/Services/WebWorker/PageHost.h

@@ -31,6 +31,7 @@ public:
     virtual void paint(Web::DevicePixelRect const&, Gfx::Bitmap&, Web::PaintOptions = {}) override;
     virtual void request_file(Web::FileRequest) override;
     virtual void schedule_repaint() override {};
+    virtual bool is_ready_to_paint() const override { return true; }
 
 private:
     explicit PageHost(ConnectionFromClient&);