TRECISION: Fix thumbnail when using ScummVM menu to save (bug #13834)

When saving from the game's control panel, we use the thumbnail that was
created before the control panel opened. But when using the ScummVM
menu, we need to generate the thumbnail as usual. Otherwise, there may
not be any thumbnail at all, causing the save to fail.

engines/trecision/logic.cpp

@@ -3988,7 +3988,9 @@ void LogicManager::handleClickControlPanel(uint16 curObj) {
 		if (_vm->_oldRoom == kRoomControlPanel)
 			break;
 		_vm->_curRoom = _vm->_obj[o00EXIT]._goRoom;
+		_vm->_controlPanelSave = true;
 		_vm->dataSave();
+		_vm->_controlPanelSave = false;
 		_vm->showInventoryName(NO_OBJECTS, false);
 		_vm->showIconName();
 		_vm->changeRoom(_vm->_obj[o00EXIT]._goRoom);

engines/trecision/metaengine.cpp

@@ -68,7 +68,13 @@ void TrecisionMetaEngine::getSavegameThumbnail(Graphics::Surface &thumb) {
 	// We are referencing g_engine here, but this should be safe, as this
 	// method is only used while the engine is running.
 	// TODO: Is there a better way to do this?
-	thumb.copyFrom(((Trecision::TrecisionEngine *)g_engine)->_thumbnail);
+
+	Trecision::TrecisionEngine *engine = (Trecision::TrecisionEngine *)g_engine;
+
+	if (engine->_controlPanelSave)
+		thumb.copyFrom(engine->_thumbnail);
+	else
+		MetaEngine::getSavegameThumbnail(thumb);
 }
 
 SaveStateDescriptor TrecisionMetaEngine::querySaveMetaInfos(const char *target, int slot) const {

engines/trecision/trecision.h

@@ -243,6 +243,7 @@ class TrecisionEngine : public Engine {
 	const ADGameDescription *_gameDescription;
 
 	Graphics::Surface _thumbnail;
+	bool _controlPanelSave = false;
 
 	uint16 _curRoom;
 	uint16 _oldRoom;