Code review for a rendering framework (C++/OpenGL)

I'm working on a framework that can be used to develop 3d applications in opengl (with the intention of updating to use vulkan in the future). The basic idea is that the user initializes the App in main(), adds an initial scene, then controls the flow of the application from the scenes they compose, abstracting away the logic required for the fixed timestep loop, model loading, gpu rendering, etc. A quick outline is as follows:

> App (singleton responsible for managing window, input, and kicking off the main loop)
	> Scene (collection of stages)
		> Stages
  			> Camera (base …