Backend/BackendGraphicsSDL: Throwing out some helper functions to its own module
Backend/BackendGraphicsSDL.h: Small headerguard fix.
Input: Small include cleanup.
Graphics/BackendGraphics: Rename GetWindowSize to GetScreenSize
Graphics/BackendGraphics: return Vec2<float> instead of Vec2<int> in GetScreenSize.
Backend/OpenGLHelper.cpp: fix some linker error with namespaces.
Adding System/BackendSystem.

do not expose SDL specific symbols to the engine more then necessary.
This makes it easier to switch underlying API