We cannot declare VipsTypeMap2Fn as opaque as this callback is used by type_map(). Instead, restore the VipsTypeMap2Fn definition and use an opaque pointer for the second argument of vips_type_map().
Details
```console
$ python3 gen_cpp_binding.py
Traceback (most recent call last):
File "/home/kleisauke/wasm-vips/build/gen_cpp_binding.py", line 295, in
generate_enums_flags('enums.cpp')
File "/home/kleisauke/wasm-vips/build/gen_cpp_binding.py", line 230, in generate_enums_flags
type_map(type_from_name('GEnum'), add_enum)
File "/home/kleisauke/.local/lib/python3.12/site-packages/pyvips/base.py", line 105, in type_map
cb = ffi.callback('VipsTypeMap2Fn', fn)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: expected a function ctype, got 'VipsTypeMap2Fn'
```
We cannot declare
VipsTypeMap2Fnas opaque as this callback is used bytype_map(). Instead, restore theVipsTypeMap2Fndefinition and use an opaque pointer for the second argument ofvips_type_map().Details
```console $ python3 gen_cpp_binding.py Traceback (most recent call last): File "/home/kleisauke/wasm-vips/build/gen_cpp_binding.py", line 295, inRegressed since: #464.