~ 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/Tut01/build.lua

@@ -9,3 +9,7 @@ EXAMPLE.Files = {
 EXAMPLE.Libraries = {
 	"NazaraSDK"
 }
+
+if Config.PlatformSDL2 then
+	table.insert(EXAMPLE.Defines, "NAZARA_PLATFORM_SDL2")
+end