Also includes migration to OpenGL (3.0 core), as imgui needs the geometry rendering. For now debug windows should be able to further use SDL_Renderer (but software-rendered), but we should change this into a more generic approach being able to render either with SDL_Renderer or imgui
[x] Add cimgui, imgui and glad as submodules and as dependencies in cmake
[x] Create containing window for all imgui windows
[x] Create generic "image" window for all windows, which render externally (PoD renderer, debug SDL_Renderer), which abstracts away the OpenGL texture/imgui window complexity
[x] Migrate PebbleWindow
[x] Migrate DebugWindow
[x] ~Create means for debug windows to be either rendering or imgui windows~ Postponed to issue #72
Fixes #30
Also includes migration to OpenGL (3.0 core), as imgui needs the geometry rendering. For now debug windows should be able to further use SDL_Renderer (but software-rendered), but we should change this into a more generic approach being able to render either with SDL_Renderer or imgui