add love.exposed and love.occluded event callbacks.

Add love.window.isOccluded.

Resolves #2004.

src/modules/window/Window.h

@@ -187,6 +187,7 @@ public:
 	virtual bool hasMouseFocus() const = 0;
 
 	virtual bool isVisible() const = 0;
+	virtual bool isOccluded() const = 0;
 
 	virtual void setMouseGrab(bool grab) = 0;
 	virtual bool isMouseGrabbed() const = 0;

src/modules/window/sdl/Window.cpp

@@ -1298,6 +1298,11 @@ bool Window::isVisible() const
 	return window && (SDL_GetWindowFlags(window) & (SDL_WINDOW_HIDDEN | SDL_WINDOW_MINIMIZED)) == 0;
 }
 
+bool Window::isOccluded() const
+{
+	return window && (SDL_GetWindowFlags(window) & SDL_WINDOW_OCCLUDED) != 0;
+}
+
 void Window::setMouseGrab(bool grab)
 {
 	mouseGrabbed = grab;

src/modules/window/sdl/Window.h

@@ -98,6 +98,7 @@ public:
 	bool hasMouseFocus() const override;
 
 	bool isVisible() const override;
+	bool isOccluded() const override;
 
 	void setMouseGrab(bool grab) override;
 	bool isMouseGrabbed() const override;

src/modules/window/wrap_Window.cpp

@@ -486,6 +486,12 @@ int w_isVisible(lua_State *L)
 	return 1;
 }
 
+int w_isOccluded(lua_State *L)
+{
+	luax_pushboolean(L, instance()->isOccluded());
+	return 1;
+}
+
 int w_getDPIScale(lua_State *L)
 {
 	lua_pushnumber(L, instance()->getDPIScale());
@@ -683,6 +689,7 @@ static const luaL_Reg functions[] =
 	{ "hasFocus", w_hasFocus },
 	{ "hasMouseFocus", w_hasMouseFocus },
 	{ "isVisible", w_isVisible },
+	{ "isOccluded", w_isOccluded },
 	{ "getDPIScale", w_getDPIScale },
 	{ "getNativeDPIScale", w_getNativeDPIScale },
 	{ "toPixels", w_toPixels },