generated abi-mode struct defs breaks unions

Created originally on Bitbucket by lunixbochs (Ryan Hileman)

If I run this union through ffi.emit_python_code, the size becomes 32 instead of 16:

struct NVGcolor {
    union {
        float rgba[4];
        struct { float r,g,b,a; };
    };
};

But the size is right before emitting (all sizes in this output should be 16):

raw ffi: 16 16
generated ffi: 32 16

In the generated ffi code, the union size is 32, suggesting the union fields were combined into a larger incorrect struct. PoC attached, repros in pypy3 and cpython with cffi 1.11.4