LibIPC+LibGfx: Pass the IPC::Decoder to decoding helpers

Instead of passing the BufferStream, pass the Decoder. I'd like to stop
using BufferStream eventually anyway, so it's good to get it out of any
API's where it's in currently.

Libraries/LibGfx/Size.h

@@ -28,6 +28,7 @@
 
 #include <AK/Forward.h>
 #include <LibGfx/Orientation.h>
+#include <LibIPC/Forward.h>
 
 namespace Gfx {
 
@@ -113,5 +114,5 @@ const LogStream& operator<<(const LogStream&, const Size&);
 }
 
 namespace IPC {
-bool decode(BufferStream&, Gfx::Size&);
+bool decode(Decoder&, Gfx::Size&);
 }