UNUSED(x) is a no-op on MSVC it lacks
the attributes GCC has for this purpose.

However C++17 added [[maybe_unused]]
which serves the same purpose which
MSVC *does* support.

This resolves the following MSVC warning:

node_geo_set_curve_handles.cc(101,16): warning C4189: 'i': local variable is initialized but not referenced

I have however no idea what our policy on C++17 attributes is as this moment, i winged the reviewers by picking the ones who ran into C++17 issues the most and threw in sergey for good measure..