Fix not being able to `TAB` change sources

Ref #55

It was not possible to change a source using `TAB` just after a new surface creation while it had the default checker source set.

src/Commands/SetNextSourceCmd.cpp

@@ -60,8 +60,12 @@ void SetNextSourceCmd::exec(){
 	}
 	
 	if(_sourceIndex == -1){
+		if(_sources.size()){
+			_nextSourceIndex = 0;
+		}else{
 			return;
 		}
+	}
 	
 	_nextSourceIndex = _sourceIndex + 1;
 	if(_nextSourceIndex >= _sources.size()){