More ISO C90 warning fixes in Clutter

More ISO C90 warning fixes in Clutter

This time, in Clutter core.

The ObjC standard library provides a type called 'id', which obviously
requires any library to either drop the useful shadowed variable warning
or stop using 'id' as a variable name.

Yes, it's almost unbearably stupid. Well, at least it's not 'index' in
string.h, or 'y2' in math.h.