PixelPaint: Add a general-purpose parallel image processing pipeline

The ImageProcessor singleton is intended to be used by all sorts of
image processing which might take some time to complete; or other
background actions. We're not using BackgroundTask here because this
system is specifically designed to work with task queues and PixelPaint
interaction; e.g. it provides common image processing tasks such as
filter application.

Userland/Applications/PixelPaint/CMakeLists.txt

@@ -40,6 +40,7 @@ set(SOURCES
     IconBag.cpp
     Image.cpp
     ImageEditor.cpp
+    ImageProcessor.cpp
     Layer.cpp
     LayerListWidget.cpp
     LayerPropertiesWidget.cpp