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/CMakeLists.txt

@@ -62,6 +62,7 @@ set(SOURCES
     Tools/MoveTool.cpp
     Tools/PenTool.cpp
     Tools/PickerTool.cpp
+    Tools/PolygonalSelectTool.cpp
     Tools/RectangleSelectTool.cpp
     Tools/RectangleTool.cpp
     Tools/SprayTool.cpp