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/ToolboxWidget.cpp

@@ -16,6 +16,7 @@
 #include "Tools/MoveTool.h"
 #include "Tools/PenTool.h"
 #include "Tools/PickerTool.h"
+#include "Tools/PolygonalSelectTool.h"
 #include "Tools/RectangleSelectTool.h"
 #include "Tools/RectangleTool.h"
 #include "Tools/SprayTool.h"
@@ -83,6 +84,7 @@ void ToolboxWidget::setup_tools()
     add_tool("zoom"sv, { 0, Key_Z }, make<ZoomTool>());
     add_tool("rectangle-select"sv, { 0, Key_R }, make<RectangleSelectTool>());
     add_tool("wand-select"sv, { 0, Key_W }, make<WandSelectTool>());
+    add_tool("polygonal-select"sv, { Mod_Shift, Key_P }, make<PolygonalSelectTool>());
     add_tool("guides"sv, { 0, Key_G }, make<GuideTool>());
     add_tool("clone"sv, { 0, Key_C }, make<CloneTool>());
 }