LibGfx: Add CMYKBitmap

This is a simple container for 2d CMYK data.

This is a new class instead of a new format in Bitmap because:
* Almost all clients of Bitmap will want to deal with RGB data
* It keeps the ARGB32 typedef working
* Bitmap already does a lot of things

The idea is that a few select places will be able to use
CMYKBitmap and a color profile to do a better CMYK -> RGB conversion
than an image decoder could do, and then store the result in a Bitmap
for later display then.

CMYKBitmap misses some of Bitmap's features, such as shared
memory support, high-dpi scaling capability, etc.

Userland/Libraries/LibGfx/Forward.h

@@ -9,6 +9,7 @@
 namespace Gfx {
 
 class Bitmap;
+class CMYKBitmap;
 class ImmutableBitmap;
 class CharacterBitmap;
 class Color;