Split the library into 6 sub-libraries

[devshgraphicsprogramming]

//{ core lib (maths, basic types, allocators, etc.)

//{ system lib (fibers, mutexes, file I/O operations) [DEPENDS: core]

//{ asset lib (importing and exporting meshes, textures and shaders incl. shader compilation) [DEPENDS: system]

//{ ui lib (window set up, software blit, joysticks, multi-touch, keyboard, etc.) [DEPENDS: system]

//{ video lib (access to Graphics API, remote rendering, etc) [DEPENDS: asset, (optional) ui]

//{ scene lib (basic rendering, culling, scene graph etc.) [DEPENDS: video, ui]

Then what used to be the IrrlichtServer target is just a specialized build of the video lib that only includes the stub graphics objects.

This opens the way to gradual ARM cross-compile and Android porting #72

[devshgraphicsprogramming]

Object initialization.

irr::core needs no global object initialization

irr::system needs IFileSystem to be created

irr::asset only needs an IAssetManager to be created (to exist separately, needs to be created manually)

irr::ui lib needs to set-up a window and some I/O listeners, so that would be the ISDL2Device creation with possibly IFileSystem passed in the constructor

irr::video needs to create our graphics device/context on GL and Vulkan or NULLDriver, so it should take an IAssetManager plus an ISDL2Device (which can be null for headless rendering)

irr::scene is basically exposing and inlining the CSceneManager

irr::system and irr::ui should take-over most of IrrlichtDevice's responsibilities, so it shall perish.

[devshgraphicsprogramming]

NBL_API_2 needs to change into 6 versions named after the namespace, so NBL_CORE_API and friends