LibGUI: ColorPicker should pick custom color from bitmap, not window

It's not always safe to access pixel data of a window's backing store
since the kernel may decide to purge it at his leisure. Fix this by
instead picking colors from the color spectrum bitmap directly instead.

Also fix up mouse event logic while we're here so it only cares about
the left mouse button

Fixes #1657.

Libraries/LibGUI/ColorPicker.cpp

@@ -72,10 +72,10 @@ private:
     CustomColorWidget();
 
     RefPtr<Gfx::Bitmap> m_custom_colors;
-    bool m_status { false };
+    bool m_being_pressed { false };
     Gfx::Point m_last_position;
 
-    void fire_event(GUI::MouseEvent& event);
+    void pick_color_at_position(GUI::MouseEvent& event);
 
     virtual void mousedown_event(GUI::MouseEvent&) override;
     virtual void mouseup_event(GUI::MouseEvent&) override;
@@ -409,22 +409,20 @@ void CustomColorWidget::clear_last_position()
     update();
 }
 
-void CustomColorWidget::fire_event(GUI::MouseEvent& event)
+void CustomColorWidget::pick_color_at_position(GUI::MouseEvent& event)
 {
-    if (!m_status)
-        return;
-
-    if (!on_pick)
+    if (!m_being_pressed)
         return;
 
     auto position = event.position();
-    if (!this->rect().contains(position)) {
+    if (!rect().contains(position))
         return;
-    }
+
+    auto color = m_custom_colors->get_pixel(position);
     m_last_position = position;
 
-    auto color = this->window()->back_bitmap()->get_pixel(position);
-    on_pick(color);
+    if (on_pick)
+        on_pick(color);
 
     update();
 }
@@ -432,21 +430,23 @@ void CustomColorWidget::fire_event(GUI::MouseEvent& event)
 void CustomColorWidget::mousedown_event(GUI::MouseEvent& event)
 {
     if (event.button() == GUI::MouseButton::Left) {
-        m_status = true;
-    } else {
-        m_status = false;
+        m_being_pressed = true;
+        pick_color_at_position(event);
     }
 }
 
 void CustomColorWidget::mouseup_event(GUI::MouseEvent& event)
 {
-    fire_event(event);
-    m_status = false;
+    if (event.button() == GUI::MouseButton::Left) {
+        m_being_pressed = false;
+        pick_color_at_position(event);
+    }
 }
 
 void CustomColorWidget::mousemove_event(GUI::MouseEvent& event)
 {
-    fire_event(event);
+    if (event.buttons() & GUI::MouseButton::Left)
+        pick_color_at_position(event);
 }
 
 void CustomColorWidget::paint_event(GUI::PaintEvent& event)