LibGfx: Add StackBlurFilter, an efficient almost gaussian blur

This adds an implementation of StackBlur which is a very efficient
blur that closely approximates a gaussian blur. It has a number of
benefits over the existing FastBoxBlurFilter:

1. It's faster (~half the pixel lookups of a single box blur pass)
2. It only requires a single pass over image to produce good results
  - (Rather than the 3 the box blur requires)
3. It only needs to allocate a buffer of `blur_radius * 2 + 1` colors
  - These easily fits on the stack for any reasonable radius

For a detailed explanation of the algorithm check out this link:
https://observablehq.com/@jobleonard/mario-klingemans-stackblur

Userland/Libraries/LibGfx/CMakeLists.txt

@@ -1,10 +1,10 @@
 set(SOURCES
     AffineTransform.cpp
     AntiAliasingPainter.cpp
-    Bitmap.cpp
-    BitmapMixer.cpp
     BMPLoader.cpp
     BMPWriter.cpp
+    Bitmap.cpp
+    BitmapMixer.cpp
     ClassicStylePainter.cpp
     ClassicWindowTheme.cpp
     Color.cpp
@@ -14,27 +14,28 @@ set(SOURCES
     Filters/ColorBlindnessFilter.cpp
     Filters/FastBoxBlurFilter.cpp
     Filters/LumaFilter.cpp
+    Filters/StackBlurFilter.cpp
     Font/BitmapFont.cpp
     Font/Emoji.cpp
     Font/FontDatabase.cpp
     Font/ScaledFont.cpp
+    Font/TrueType/Cmap.cpp
     Font/TrueType/Font.cpp
     Font/TrueType/Glyf.cpp
-    Font/TrueType/Cmap.cpp
     Font/Typeface.cpp
     Font/WOFF/Font.cpp
     GIFLoader.cpp
     ICOLoader.cpp
     ImageDecoder.cpp
     JPGLoader.cpp
-    Painter.cpp
-    Palette.cpp
-    Path.cpp
     PBMLoader.cpp
     PGMLoader.cpp
     PNGLoader.cpp
     PNGWriter.cpp
     PPMLoader.cpp
+    Painter.cpp
+    Palette.cpp
+    Path.cpp
     Point.cpp
     QOILoader.cpp
     QOIWriter.cpp