Issue 5217: Fix sorting order without outside-staff-priority

Issue 5217: Fix sorting order without outside-staff-priority

If the two Grobs have no outside-staff-priority, the compare function
staff_priority_less() would relate the two pointers. This may lead to
changing sorting orders in subsequent runs, apparently resulting in
"random" positions in the regression tests rest-dot-position.ly and
sometimes merge-rests-engraver.ly.
Solve this by keeping the original order in the vector:
 * Mark two Grobs without outside-staff-priority as being equal by
   always returning false (none is less than the other), and
 * use vector_stable_sort() to keep equal items in their relation.

[Dan Eble, 2019-11-05 19:23:40]

THANK YOU

I applied this on top of master, ran `make test-baseline` and then `make check`
and there were no differences other than the expected test-output-distance case.

LGTM

[Carl, 2019-11-05 19:33:21]

LGTM.  Nice work!

Carl

[lilypond-pkx, 2019-11-06 09:33:45]

Interestingly (or not) I did 3 patch runs on this patch and 2 out of 3 times I
still got the reg test showing.

I have some time today so I'll see if I can spot anything and  update
accordingly.

Jams

[hahnjo, 2019-11-06 09:36:39]

On 2019/11/06 09:33:45, lilypond-pkx wrote:
> Interestingly (or not) I did 3 patch runs on this patch and 2 out of 3 times I
> still got the reg test showing.
> 
> I have some time today so I'll see if I can spot anything and  update
> accordingly.
> 
> Jams

Compared to which baseline, run with master or with this patch already applied?
If with master, you'll have the random positions in the baseline...
If with this patch applied, I'll try later if I can reproduce on my system.