Add support for set_formspec_prepend in main menu (#8611)

doc/menu_lua_api.txt

@@ -79,6 +79,8 @@ core.explode_table_event(string) -> table
 core.explode_textlist_event(string) -> table
 ^ returns e.g. {type="CHG", index=1}
 ^ type: "INV" (no row selected), "CHG" (selected) or "DCL" (double-click)
+core.set_formspec_prepend(formspec)
+^ string to be added to every mainmenu formspec, to be used for theming.
 
 GUI:
 core.set_background(type, texturepath,[tile],[minsize])

src/gui/guiEngine.cpp

@@ -389,6 +389,15 @@ void GUIEngine::cloudPostProcess()
 	}
 }
 
+/******************************************************************************/
+void GUIEngine::setFormspecPrepend(const std::string &fs)
+{
+	if (m_menu) {
+		m_menu->setFormspecPrepend(fs);
+	}
+}
+
+
 /******************************************************************************/
 void GUIEngine::drawBackground(video::IVideoDriver *driver)
 {
@@ -610,4 +619,3 @@ unsigned int GUIEngine::queueAsync(const std::string &serialized_func,
 {
 	return m_script->queueAsync(serialized_func, serialized_params);
 }
-

src/gui/guiEngine.h

@@ -221,6 +221,8 @@ private:
 	/** script basefolder */
 	std::string              m_scriptdir = "";
 
+	void setFormspecPrepend(const std::string &fs);
+
 	/**
 	 * draw background layer
 	 * @param driver to use for drawing

src/script/lua_api/l_mainmenu.cpp

@@ -106,6 +106,21 @@ int ModApiMainMenu::l_update_formspec(lua_State *L)
 	return 0;
 }
 
+/******************************************************************************/
+int ModApiMainMenu::l_set_formspec_prepend(lua_State *L)
+{
+	GUIEngine *engine = getGuiEngine(L);
+	sanity_check(engine != NULL);
+
+	if (engine->m_startgame)
+		return 0;
+
+	std::string formspec(luaL_checkstring(L, 1));
+	engine->setFormspecPrepend(formspec);
+
+	return 0;
+}
+
 /******************************************************************************/
 int ModApiMainMenu::l_start(lua_State *L)
 {
@@ -1041,6 +1056,7 @@ int ModApiMainMenu::l_do_async_callback(lua_State *L)
 void ModApiMainMenu::Initialize(lua_State *L, int top)
 {
 	API_FCT(update_formspec);
+	API_FCT(set_formspec_prepend);
 	API_FCT(set_clouds);
 	API_FCT(get_textlist_index);
 	API_FCT(get_table_index);

src/script/lua_api/l_mainmenu.h

@@ -104,6 +104,8 @@ private:
 
 	static int l_update_formspec(lua_State *L);
 
+	static int l_set_formspec_prepend(lua_State *L);
+
 	static int l_get_screen_info(lua_State *L);
 
 	//filesystem