~ Initial SDL2 implementation

Limitation
- Dependent projects need to set NAZARA_PLATFORM_SDL2 if nazara has been build with SDL2 since OpenGL.hpp (and maybe some other headers) exposes platform details
- SDL2 window doesn't supports async window since the API isn't fitting for now
- Contexts parameters can't be changed until we close all the SDL windows (SDL limitation)

build/config.lua

@@ -24,3 +24,6 @@ ServerMode = false
 
 -- Builds modules as one united library (useless on POSIX systems)
 UniteModules = false
+
+-- Use SDL2 platform
+PlatformSDL2 = true