~ 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)

examples/MeshInfos/build.lua

@@ -12,3 +12,6 @@ EXAMPLE.Libraries = {
 	"NazaraUtility"
 }
 
+if Config.PlatformSDL2 then
+	table.insert(EXAMPLE.Defines, "NAZARA_PLATFORM_SDL2")
+end