Add C bindings for the libf3d

[mwestphal]

Is your feature request related to a problem? Please describe.
Currently, the libf3d has a C++11 API and provides bindings in python and javascript (experimental).

Providing C bindings would open libf3d usage to many other languages, especially Julia and Rust.

Let's see if there is an interest for this. If you have interest, please do not hesitate to comment on this issue.

[kidharb]

@mwestphal please assign this to me

[Meakk]

Here's my opinion on this topic:

• Functions of the form instance.function(arg1, arg2) should look like function(instance, arg1, arg2)
• Return references should become argument pointers
• Returned std::vector should become int* size and T* ptr arguments

We should add the bindings directly in the public headers where it's appropriate, it should look like:

#ifdef __cplusplus

/* C++ class definition here*/

extern "C" {
#endif

/* C bindings definition here*/

#ifdef __cplusplus
}
#endif

[parkerstafford]

I'll work on this!

[mwestphal]

Need any help @parkerstafford ? :)