Previously, when converting from a Cairo struct to a GIArgument, the
"may_be_null" parameter would be ignored and a null pointer was always
accepted (and then would probably crash, although this seems to be an
unlikely case as we never got any bug reports of it.)

Instead, we do a null check, and throw a JS exception if needed, in the
foreign struct conversion callbacks in the various Cairo types.

Requires exposing a previously static function in arg.cpp so that the
thrown exceptions can have more useful messages.

This bug was found with -Wunused-parameter.