WindowServer: Add support for alpha channel based hit testing

This enables implementing non-rectangular window shapes, including
non-rectangular window frames.

Userland/Libraries/LibGUI/Window.cpp

@@ -147,6 +147,7 @@ void Window::show()
         m_frameless,
         m_accessory,
         m_opacity_when_windowless,
+        m_alpha_hit_threshold,
         m_base_size,
         m_size_increment,
         m_resize_aspect_ratio,
@@ -673,6 +674,20 @@ void Window::set_opacity(float opacity)
     WindowServerConnection::the().send_sync<Messages::WindowServer::SetWindowOpacity>(m_window_id, opacity);
 }
 
+void Window::set_alpha_hit_threshold(float threshold)
+{
+    if (threshold < 0.0f)
+        threshold = 0.0f;
+    else if (threshold > 1.0f)
+        threshold = 1.0f;
+    if (m_alpha_hit_threshold == threshold)
+        return;
+    m_alpha_hit_threshold = threshold;
+    if (!is_visible())
+        return;
+    WindowServerConnection::the().send_sync<Messages::WindowServer::SetWindowAlphaHitThreshold>(m_window_id, threshold);
+}
+
 void Window::set_hovered_widget(Widget* widget)
 {
     if (widget == m_hovered_widget)

Userland/Libraries/LibGUI/Window.h

@@ -72,6 +72,9 @@ public:
     void set_opacity(float);
     float opacity() const { return m_opacity_when_windowless; }
 
+    void set_alpha_hit_threshold(float);
+    float alpha_hit_threshold() const { return m_alpha_hit_threshold; }
+
     WindowType window_type() const { return m_window_type; }
     void set_window_type(WindowType);
 
@@ -238,6 +241,7 @@ private:
     RefPtr<Gfx::Bitmap> m_custom_cursor;
     int m_window_id { 0 };
     float m_opacity_when_windowless { 1.0f };
+    float m_alpha_hit_threshold { 0.0f };
     RefPtr<Widget> m_main_widget;
     WeakPtr<Widget> m_focused_widget;
     WeakPtr<Widget> m_global_cursor_tracking_widget;

Userland/Libraries/LibGfx/ClassicWindowTheme.h

@@ -52,6 +52,7 @@ public:
     {
         return compute_frame_colors(state, palette).uses_alpha();
     }
+    virtual float frame_alpha_hit_threshold(WindowState) const override { return 1.0f; }
 
 private:
     struct FrameColors {

Userland/Libraries/LibGfx/WindowTheme.h

@@ -63,6 +63,7 @@ public:
     virtual Vector<IntRect> layout_buttons(WindowType, const IntRect& window_rect, const Palette&, size_t buttons) const = 0;
     virtual bool is_simple_rect_frame() const = 0;
     virtual bool frame_uses_alpha(WindowState, const Palette&) const = 0;
+    virtual float frame_alpha_hit_threshold(WindowState) const = 0;
 
 protected:
     WindowTheme() { }