skybox: added moon and stars

src/window/Window.h

@@ -14,6 +14,10 @@ class ImageData;
 struct DisplaySettings;
 struct GLFWmonitor;
 
+enum class blendmode {
+    normal, addition
+};
+
 class Window {
 	static GLFWwindow* window;
 	static DisplaySettings* settings;
@@ -53,6 +57,8 @@ public:
     static const char* getClipboardText();
 	static DisplaySettings* getSettings();
 
+    static void setBlendMode(blendmode mode);
+
 	static glm::vec2 size() {
 		return glm::vec2(width, height);
 	}