RefUpdate1<V> method
Allocates o into slot '1', calls fn with the pointer, then
syncs native memory back into o via setCFunc.
Use this instead of Ref1 when the C function writes into the struct and
you want the mutations reflected in o after the call.
Implementation
V RefUpdate1<V>(X? o, V Function(Pointer<T> p) fn) => _RefUpdate$(o, fn, Ref1);