refactor: split into static libraries and restructure scenes

- Create cbt_opengl static library for OpenGL abstraction
- Create cbt_scene static library for base scene class
- Create scenes_cube static library for cube scene
- Move scene base to cbt/ (utility), scenes to root scenes/
- Link libraries in dependency chain: cuber -> scenes -> cbt_scene -> cbt_opengl

cbt/scene.cpp

@@ -0,0 +1,13 @@
+#include "cbt/scene.hpp"
+
+namespace cbt {
+
+auto scene::init() -> bool {
+    return true;
+}
+
+auto scene::update(float) -> void {}
+
+auto scene::render() -> void {}
+
+}