AK: Add trivial structure validation to SharedBuffer

If we're sharing buffers, we only want to share trivial structures
as anything else could potentially share internal pointers, which
most likely is going to cause problems due to different address
spaces.

Fix the GUI::SystemTheme structure, which was not trivial, which
is now caught at compile time.

Fixes #3650

Libraries/LibImageDecoderClient/Client.cpp

@@ -57,7 +57,7 @@ RefPtr<Gfx::Bitmap> Client::decode_image(const ByteBuffer& encoded_data)
         return nullptr;
     }
 
-    memcpy(encoded_buffer->data(), encoded_data.data(), encoded_data.size());
+    memcpy(encoded_buffer->data<void>(), encoded_data.data(), encoded_data.size());
 
     encoded_buffer->seal();
     encoded_buffer->share_with(server_pid());