Improve CVideo::get_available_resolutions.

If there is a main window use that else fall back to the original code.

Based on a patch by AI0867.

src/video.cpp

@@ -559,12 +559,22 @@ sdl::twindow *CVideo::get_window()
 
 #endif
 
+#if SDL_VERSION_ATLEAST(2, 0, 0)
+static int sdl_display_index()
+{
+	if(window) {
+		return SDL_GetWindowDisplayIndex(*window);
+	}
+	return 0;
+}
+#endif
+
 #if SDL_VERSION_ATLEAST(2, 0, 0)
 std::vector<std::pair<int, int> > CVideo::get_available_resolutions()
 {
 	std::vector<std::pair<int, int> > result;
 
-	const int modes = SDL_GetNumDisplayModes(0);
+	const int modes = SDL_GetNumDisplayModes(sdl_display_index());
 	if(modes <= 0) {
 		std::cerr << "No modes supported\n";
 		return result;