PixelPaint: Add a Mask class

The Mask class represents an opacity mask over a rectangular section
of an image, linking every pixel to an alpha value ranging from 0 (not
selected) to 255 (fully selected). "Partially selected" pixels can be
used to simulate anti-aliased curves.

This class will be used as the basis for the new non-rectangular
selection feature.

Userland/Applications/PixelPaint/CMakeLists.txt

@@ -28,6 +28,7 @@ set(SOURCES
     PixelPaintWindowGML.h
     RectangleTool.cpp
     RectangleSelectTool.cpp
+    Mask.cpp
     Selection.cpp
     SprayTool.cpp
     ToolboxWidget.cpp