Add initial marker in frame 0 when loading older tasprojs (#2693)

else this would make the first created marker take slot 1 and be uneditable and undeletable, which isn't good

src/BizHawk.Client.Common/movie/tasproj/TasMovie.IO.cs

@@ -74,6 +74,7 @@ namespace BizHawk.Client.Common
 				else
 				{
 					Session.PopupMessage("The current .tasproj is not compatible with this version of BizHawk! .tasproj features failed to load.");
+					Markers.Add(0, StartsFromSavestate ? "Savestate" : "Power on");
 				}
 			}
 		}