Project structure not exactly a library.

The current code works more as an executable which is just a model loader. What do we want to do?

I personally see two options we can do.

1. Not make this a rendering library, but just a path tracer. (Which can make pretty things), and maybe turn it into some type of SDK in the **far** future...
2. Focus on making this a library.

I would personally go for option one, as we do not have the most amount of features currently. And it would be a much better idea to make the images pretty first, and then worry about making it a library. Versus having to do both at the same time.

Just opening because this is something that needs to be discussed, but it's yet to be something **serious**, but something to mention.

No files, tests, or concrete entry points are named. Review the current C++ executable and model-loader structure, then look for a maintainer decision on whether the project should remain a path tracer or become a library; done requires an agreed direction and concrete implementation scope.

由索引模型根据 Issue 内容生成。