LibIPC+IPCCompiler: Add IPC::Decoder, let classes decode themselves

This shaves ~5 seconds off of a full build, not too bad. Also it just
seems nicer to push this logic out to classes. It could be better but
it's a start. :^)

Libraries/LibGfx/Size.cpp

@@ -24,6 +24,7 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#include <AK/BufferStream.h>
 #include <AK/String.h>
 #include <LibGfx/Size.h>
 
@@ -40,3 +41,19 @@ const LogStream& operator<<(const LogStream& stream, const Size& value)
 }
 
 }
+
+namespace IPC {
+
+bool decode(BufferStream& stream, Gfx::Size& size)
+{
+    int width;
+    int height;
+    stream >> width;
+    stream >> height;
+    if (stream.handle_read_failure())
+        return false;
+    size = { width, height };
+    return true;
+}
+
+}