PixelPaint: Introduce a vectorscope

Vectorscopes are a standard tool in professional video/film color
grading. *Very* simply, the Vectorscope shows image colors with hue as
the angle and saturation as the radius; brightness for each point in the
scope is determined by the number of "color vectors" at that point. More
specifically, the Vectorscope shows a 2D UV histogram of the image,
where U and V are the chroma ("color") channels of the image.

Co-authored-by: MacDue <macdue@dueutil.tech>

Userland/Applications/PixelPaint/CMakeLists.txt

@@ -72,6 +72,7 @@ set(SOURCES
     Tools/Tool.cpp
     Tools/WandSelectTool.cpp
     Tools/ZoomTool.cpp
+    VectorscopeWidget.cpp
     main.cpp
     )