Improve horizontal spacing performance by up to 70% if systems are locked

[mike-spa]

Don't know how this didn't occur to me before. The main thing that's expensive about horizontal spacing is that we need to update it measure by measure as we add measures to the system, in order to update the resulting system width and therefore decide how many measures to fit (i.e. where to break the line). But if a system is locked that's not necessary, because the line breaking decision is not up to us, which means we can skip all the intermediate calculations and just continue adding measures until the last measure of the current lock. This saves approximately 2/3 of the computation time.

Horizontal spacing only accounts for a relatively small part of the whole layout computation time, so it's not like this is going make a massive difference in the user experience. Still, in my test file (orchestral piece with all parts generated and all systems locked both in score and in parts) this saved 9-10% of the total layout time, not too bad.