WindowServer: Add a more generic mechanism for animations

This patch adds the WindowServer::Animation class, which represents
a simple animation driven by the compositor.

An animation has a length (in milliseconds) and two hooks:

- on_update: called whenever the animation should render something.
- on_stop: called when the animation is finished and/or stopped.

This patch also ports the window minimization animation to this new
mechanism. :^)

Userland/Services/WindowServer/Animation.cpp

@@ -0,0 +1,53 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "Animation.h"
+#include "Compositor.h"
+#include <AK/Badge.h>
+
+namespace WindowServer {
+
+Animation::Animation()
+{
+    Compositor::the().register_animation({}, *this);
+}
+
+Animation::~Animation()
+{
+    Compositor::the().unregister_animation({}, *this);
+}
+
+void Animation::set_length(int length_in_ms)
+{
+    m_length = length_in_ms;
+}
+
+void Animation::start()
+{
+    m_running = true;
+    m_timer.start();
+}
+
+void Animation::stop()
+{
+    m_running = false;
+    if (on_stop)
+        on_stop();
+}
+
+void Animation::update(Badge<Compositor>, Gfx::Painter& painter, Screen& screen, Gfx::DisjointRectSet& flush_rects)
+{
+    int elapsed_ms = m_timer.elapsed();
+    float progress = min((float)elapsed_ms / (float)m_length, 1.0f);
+
+    if (on_update)
+        on_update(progress, painter, screen, flush_rects);
+
+    if (progress >= 1.0f)
+        stop();
+}
+
+}