LibGfx: Implement box sampling image scaling

Box sampling is a scaling algorithm that averages all the pixels that
form the source for the target pixel. For example, if you would resize a
9x9 image to 3x3, each target pixel would encompass a 3x3 pixel area in
the source image.

Box sampling is a near perfect scaling algorithm for downscaling. When
upscaling with this algorithm, the result is similar to nearest neighbor
or smooth pixels.

Userland/Libraries/LibGfx/Painter.h

@@ -45,6 +45,7 @@ public:
         NearestNeighbor,
         SmoothPixels,
         BilinearBlend,
+        BoxSampling,
         None,
     };