FontSpec is completely different per platform; not only is the
structure type different, not only are the behind-the-scenes
implementations of copy and free functions different, but even the API
of the constructor function is different. Cross-platform code can't
construct a FontSpec at all. This makes it hard to write a
cross-platform test program that works with them.
So, as a nasty bodge, I'll allow test programs to #define
SUPERSEDE_FONTSPEC_FOR_TESTING before including putty.h. Then they can
provide their own definition of FontSpec, and they also take
responsibility for superseding all the other functions that work with
one.
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