WIP basic scene-graph with frustrum culling for Raylib

• frustum.h : contains basic frustum functions ;
• rnodes.h : contains API to create scene-graph / node-graph manually ;
• rscenegraph.h : WIP

STATUS:

• A little busy fixing some issues on an other repo, but i'll come back finishing this project ASAP. that was raysan5/raylib#4151
• Raylib is a nice and seductive little project, but I'm currently evaluating migrating to Godot and GDextension to avoid wasting time and energy trying to workaround heavy code-debt and maintainance inertie.