Use exact type for ft_gzip_zalloc and ft_gzip_free

While a function pointer may be cast to another function pointer type, it is required to cast the function pointer back to the original function pointer type before calling it. If a parameter is a pointer the exact pointer type is required. Using a pointer to a different underlying type is technically undefined behavior. The wrapper functions ft_gzip_alloc and ft_gzip_free took FT_Memory (a FT_MemoryRec_) instead of voidpf (void) so when gzip calls these callbacks through alloc_func or free_func it invokes undefined behavior. On most platforms this works out as expected, but newer undefined behavior detectors and targets like wasm can detect this and will produce an error.

Update the signatures of ft_gzip_zalloc and ft_gzip_free to exactly match alloc_func and free_func. Internally cast the void* opaque pointer to FT_Memory memory.