topic/snacman#136 secondary window

[Adnn]

closes #136.

• Show a trivial a second window, naively added in main().
• Start implementing a secondary view, as a top-down ortho camera.
• Take the draw FBO as parameter of Graph::render().
• Decouple TheGraph render size from default framebuffer size.
• Get the secondary window resize behaviour, at the cost of many hacks.
• fixup
• Abstract the environment texture repo in Environment class.
• fixup
• Document (TODOs) some remaining problems with the graph design.
• Separate the blitting class from the secondary view.
• Move debug rendering from TheGraph to ViewerApplication.
• Separate the primary view in a distinct struct, out of ViewerApp.
• Move secondary view in ViewerApplication, handle window via GUI.
• Draw debug in the secondary view, add primary camera basis.
• Accept the initial orbital pose on CameraSystem construction.
• Prototype showing a separate ImGui context in secondary window.
• Add debug camera controls to secondary window GUI.
• Fix bug in Camera projection change detection.
• Show camera view frustum via debug drawer.
• Upgrade dependencies.