Demos: Add ability to use scroll wheel in starfield

This change adds the ability to use the scroll wheel to change
the speed in starfield.

Userland/Demos/Starfield/Starfield.cpp

@@ -46,6 +46,7 @@ class Starfield final : public Desktop::Screensaver {
  virtual void paint_event(GUI::PaintEvent&) override;
  virtual void timer_event(Core::TimerEvent&) override;
  virtual void keydown_event(GUI::KeyEvent&) override;
+ virtual void mousewheel_event(GUI::MouseEvent&) override;
 
  Vector<Coordinate> m_stars;
  int m_sweep_plane = 2000;
@@ -89,6 +90,16 @@ void Starfield::keydown_event(GUI::KeyEvent& event)
  }
 }
 
+void Starfield::mousewheel_event(GUI::MouseEvent& event)
+{
+ if (event.wheel_delta_y() == 0)
+ return;
+
+ m_speed += event.wheel_delta_y() > 0 ? -1 : 1;
+ if (m_speed < 1)
+ m_speed = 1;
+}
+
 void Starfield::paint_event(GUI::PaintEvent& event)
 {
  GUI::Painter painter(*this);