DebugTools: suggest member slicing instead of a cast.

doc/snippets/DebugTools-gl.cpp

@@ -216,7 +216,7 @@ ImageView2D expected{PixelFormat::RGB8Unorm, {}};
 /* [CompareImage-pixels-rgb] */
 Image2D image = fb.read(fb.viewport(), {PixelFormat::RGBA8Unorm});
 
-CORRADE_COMPARE_AS(Containers::arrayCast<Color3ub>(image.pixels<Color4ub>()),
+CORRADE_COMPARE_AS(image.pixels<Color4ub>().slice(&Color4ub::rgb),
     "expected.png", DebugTools::CompareImageToFile);
 /* [CompareImage-pixels-rgb] */
 }

src/Magnum/DebugTools/CompareImage.h

@@ -322,8 +322,8 @@ data flipped upside down:
 
 For a different scenario, imagine you're comparing data read from a framebuffer
 to a ground truth image. On many systems, internal framebuffer storage has to
-be four-component; however your if your ground truth image is just
-three-component you can cast the pixel data to just a three-component type:
+be four-component; however your if your ground truth image is three-component
+you can slice the pixel data to just the RGB channels:
 
 @snippet DebugTools-gl.cpp CompareImage-pixels-rgb