imcontext: handle >16bit keysyms
I've recently become aware that there are keyboard layouts which use Unicode keysyms for combining marks, which is a (nice, imo) alternative to dead keys. Those are >16bit.
At least our Compose table format currently assumes 16bit keysyms, but we should systematically check that these keysyms work everywhere.
At the very least, we should not silently truncate Unicode keysyms to 16bit.