PixelPaint: Add a Bloom filter

A bloom filter creates fringes around bright areas in the image
mimicking the behavior of real-world cameras.
It gets its own category "Artistic" in the Filter Gallery since its not
one filter per se but a combination of multiple.

The filter works as follows:
- Get only the light areas (above a threshold) of the image
- Blur that image
- Compose onto the original image

Userland/Applications/PixelPaint/CMakeLists.txt

@@ -17,6 +17,7 @@ set(SOURCES
     FilterGallery.cpp
     FilterGalleryGML.h
     FilterModel.cpp
+    Filters/Bloom.cpp
     Filters/BoxBlur3.cpp
     Filters/BoxBlur5.cpp
     Filters/Filter.cpp