Allow to define stretch mode to title screen:

```
SetTitleScreens(
  {Image = "ui/title.png", StretchMode = "none"}, -- Keep original image size
  {Image = "ui/title.png", StretchMode = "keep-ratio"}, -- resize to touch border, and keep original size ratio
  {Image = "ui/title.png", StretchMode = "stretch"} -- (default) resize to the full windows
)
```

src/include/title.h

@@ -55,6 +55,15 @@ class TitleScreenLabel
 	int Flags = 0;
 };
 
+enum class EStretchMode
+{
+	None,
+	KeepRatio,
+	Stretch
+};
+
+std::optional<EStretchMode> StretchModeFromString(std::string_view);
+
 class TitleScreen
 {
 public:
@@ -64,11 +73,11 @@ class TitleScreen
 	void ShowTitleImage();
 
 private:
-	void ShowLabels();
+	void ShowLabels() const;
 public:
 	fs::path File;
 	std::string Music;
-	bool StretchImage = true;
+	EStretchMode StretchMode = EStretchMode::Stretch;
 	int Timeout = 0;
 	int Editor = 0;
 	std::vector<std::unique_ptr<TitleScreenLabel>> Labels;

src/stratagus/title.cpp

@@ -42,7 +42,37 @@ std::vector<std::unique_ptr<TitleScreen>> TitleScreens; /// Title screens to sho
 static bool WaitNoEvent;             /// Flag got an event
 extern std::string CliMapName;
 
-void TitleScreen::ShowLabels()
+static void ApplyStretchMode(CGraphic &g, EStretchMode mode, int width, int height)
+{
+	switch (mode) {
+		case EStretchMode::None: break;
+		case EStretchMode::KeepRatio:
+		{
+			if (g.Width * height < width * g.Height) {
+				g.Resize(g.Width * height / g.Height, height);
+			} else {
+				g.Resize(width, g.Height * width / g.Width);
+			}
+			break;
+		}
+		case EStretchMode::Stretch: g.Resize(width, height); break;
+	}
+}
+
+std::optional<EStretchMode> StretchModeFromString(std::string_view s)
+{
+	if (s == "none") {
+		return EStretchMode::None;
+	} else if (s == "keep-ratio") {
+		return EStretchMode::KeepRatio;
+	} else if (s == "stretch") {
+		return EStretchMode::Stretch;
+	} else {
+		return std::nullopt;
+	}
+}
+
+void TitleScreen::ShowLabels() const
 {
 	for (auto &titleScreenlabel : this->Labels) {
 		if (!titleScreenlabel->Font) {
@@ -85,9 +115,7 @@ void TitleScreen::ShowTitleImage()
 
 	auto g = CGraphic::New(this->File.string());
 	g->Load();
-	if (this->StretchImage) {
-		g->Resize(Video.Width, Video.Height);
-	}
+	ApplyStretchMode(*g, this->StretchMode, Video.Width, Video.Height);
 
 	int timeout = this->Timeout ? this->Timeout * CYCLES_PER_SECOND : -1;
 

src/ui/script_ui.cpp

@@ -377,6 +377,13 @@ static int CclSetTitleScreens(lua_State *l)
 			const std::string_view value = LuaToString(l, -2);
 			if (value == "Image") {
 				TitleScreens[j]->File = LuaToString(l, -1);
+			} else if (value == "StretchMode") {
+				const std::string_view value = LuaToString(l, -1);
+				if (auto stretchMode = StretchModeFromString(value)) {
+					TitleScreens[j]->StretchMode = *stretchMode;
+				} else {
+					LuaError(l, "Unknown StretchMode %s", value.data());
+				}
 			} else if (value == "Music") {
 				TitleScreens[j]->Music = LuaToString(l, -1);
 			} else if (value == "Timeout") {