Add demo shader and spinning colored cube

Rsconscript

@@ -13,8 +13,10 @@ env "app" do |env|
   sources = dirs.reduce([]) do |result, dir|
     result + glob("#{dir}/**/*.{d,c}")
   end
+  sources += glob("libs/gl3n/gl3n/**/*.{d,c}")
   env["D_IMPORT_PATH"] += dirs
-  env["DFLAGS"] += %w[--d-version=BindBC_Static --d-version=SDL_208]
+  env["D_IMPORT_PATH"] += ["libs/gl3n"]
+  env["DFLAGS"] += %w[--d-version=BindBC_Static --d-version=SDL_208 -g]
   env["LDFLAGS"] += %w[-L-lfreetype -L-lSDL2]
   env.Program("^/app", sources)
 end