Skybox and Frame buffer

I've been away for awhile, but meanwhile, the skybox implementation was completed for both rendering API. Despite that user cannot configure the skybox material at the moment, I believe it will be available sooner or later during the development process. I've also moved the game layer from client project to YUGEN's core project. And assuming there's only a single scene and renderer inside an application instance for now. Ensuring every scene has a option to clear with specified color or skybox. This may not be the final structure, but it's settled for now.

In this update, frame buffer is also introduced to the engine for both rendering API. With frame buffers, scene preview no longer needs to render on top of the ImGui layer. Instead, it is rendered into a frame buffer and being told to render as texture image in ImGui layer. Another benefit for such is post-processing. Hopefully it will be available in the future (By using one post-processing shader with different preset effect filter functions).

Another important change was shader header. I prototyped it using .json file extension for storing shader resources information. For more detail, you can read the README.md under res/shaders in Sandbox project directory. By doing that, it reduced parsing mistakes in shaders and it can works for both HLSL and GLSL resource declaration. But at the moment, I have no idea why I used .json as the extension (XML would work too but JSON looks cleaner for me :P). If you have a better idea, feel free to put forward your suggestion!

Of course every updates came with new issues, this one was no exception. After implementing frame buffer, I've noticed whenever resizing the window would lead to weird behavior in DirectX in MSWindows. Besides, Mac build seems a little off with the scene preview too. I'll look into that, perhaps that's only something to do with the viewport or the frame buffer dimension. Thank you for reading to the end! <3