anim-controller: fix transitions sorting / minimize code duplication

[kungfooman]

A sort function doesn't act on true/false, so this never sorted anything (copy/paste test for F12/DevTools):

transitions = [
  {priority: 10},
  {priority: 50},
  {priority: 1},
  {priority: 2},
  {priority: 100},
  {priority: 3},
];
transitions.sort(function (a, b) {
    return a.priority < b.priority;
});
console.log(transitions.map(_ => _.priority));

Result: [10, 50, 1, 2, 100, 3] (its fixable by replacing < with -)

And then priority sorting is used in multiple places, so this PR reduces the code size/repetition of the same sorting function by just reusing functions (imo saving time and hopefully less bugs)

I confirm I have read the contributing guidelines and signed the Contributor License Agreement.

[willeastcott]

Why does cmpPriority need to be exported? Should it be internal to this module?

[kungfooman]

I just like to keep things public for potential reuse, but technically it can be private (since its not being used elsewhere)

[mvaligursky]

please make this not exported, and we can merge it in for the next release

[mvaligursky]

I'll merge this and hide cmpPriority in separate PR.

[mvaligursky]

#4077

[willeastcott]

LGTM. However, @mvaligursky and @ellthompson should approve before merging since this affects both animation and graphics subsystems.

[mvaligursky]

I looks great, but I would not mind some simple test of this in the test/core folder