PixelPaint: Add Polygonal Select Tool

Polygonal selection tool allows for the drawing of any arbitrary
polygonal shape. It tracks clicked points in a vector, upon double
clicking we finalize the polygon and generate the selection mask. The
user can press the escape key during selection to cancel.

The mask is generated as follows:

- First we calculate the size of the bounding rect needed to hold the
  polygon
- We add 2 pixels to height/width to allow us a 1 pixel border, the
  polygon will be centered in this bitmap
- Draw the polygon into the bitmap via Gfx::Painter, making sure to
  connect final polygon point to the first to ensure an enclosed shape
- Generate a selection mask the size of the bitmap, with all pixels
  initially selected
- Perform a flood fill from (0,0) which is guaranteed to be outside the
  polygon
- For every pixel reached by the flood fill, we clear the selected pixel
  from the selection mask
- Finally we merge the selection mask like other selection tools.

Userland/Applications/PixelPaint/Tools/PolygonalSelectTool.h

@@ -0,0 +1,41 @@
+/*
+ * Copyright (c) 2022, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include "../Selection.h"
+#include "Tool.h"
+#include <AK/Vector.h>
+#include <LibGUI/Widget.h>
+
+namespace PixelPaint {
+
+class PolygonalSelectTool final : public Tool {
+public:
+    PolygonalSelectTool() = default;
+    virtual ~PolygonalSelectTool() = default;
+    virtual void on_doubleclick(Layer*, MouseEvent& event) override;
+    virtual void on_mousedown(Layer*, MouseEvent& event) override;
+    virtual void on_mousemove(Layer*, MouseEvent& event) override;
+    virtual void on_keydown(GUI::KeyEvent&) override;
+    virtual void on_second_paint(Layer const*, GUI::PaintEvent&) override;
+    virtual GUI::Widget* get_properties_widget() override;
+    virtual Variant<Gfx::StandardCursor, NonnullRefPtr<Gfx::Bitmap>> cursor() override { return Gfx::StandardCursor::Crosshair; }
+    virtual Gfx::IntPoint point_position_to_preferred_cell(Gfx::FloatPoint const& position) const override;
+
+private:
+    virtual void flood_polygon_selection(Gfx::Bitmap&, Gfx::IntPoint);
+    virtual void process_polygon();
+    virtual StringView tool_name() const override { return "Polygonal Select Tool"sv; }
+
+    RefPtr<GUI::Widget> m_properties_widget;
+    Selection::MergeMode m_merge_mode { Selection::MergeMode::Set };
+    bool m_selecting { false };
+    Gfx::IntPoint m_last_selecting_cursor_position;
+    Vector<Gfx::IntPoint> m_polygon_points {};
+};
+
+}