The GDNative C API gets passed to libraries in a struct of function
pointers. To provide stable binary compatibility, each extension not
part of the core API is separated into its own sub-struct.
These structs aren't meant to be changed in order to keep binary
compatibility.
In case of an API extension, the structs include a `next` pointer
which can point to a new struct with additional function pointers.
Godot's build system generates the API structs automatically at
build time, but so far there has no support for the mentioned `next`
pointers.
This commit changes the API struct generation in such a way that code
that used previous headers will compile without problem with the new
headers.
The new extension-extensions (weird name, but that's what it is) get
generated recursively and include the version in the struct-name.
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