[varStore] Improve optimize algorithm

[behdad]

This was always supposed to be how it works, but was missed in the initial implementation apparently.

I should probably add extensive tests for the optimization algorithm...

[behdad]

Saves 408 bytes building NotoSansArabic.

[anthrotype]

hm in the absence of a test that touches those lines, it's a bit too hard for me to reivew so I'll pass. feel free to merge, this is your code

[behdad]

Adding Jany and Nikolaus in case one of them wants to read and understand the algorithm. I'm willing to add more comments. And yes I'll think about adding tests.

[behdad]

I documented the algorithm.

[rsheeter]

The doc comment is most appreciated. +1 to tests.

Saves 408 bytes building NotoSansArabic.

Are there cases where it saves more?

[behdad]

The doc comment is most appreciated. +1 to tests.

Saves 408 bytes building NotoSansArabic.

Are there cases where it saves more?

In NotoSans it saved 2kb, but that's 0.1%. I don't expect it to be much; in the 1kb to 2kb range, but hey, it's free savings.

[behdad]

In NotoSans it saved 2kb

Another bugfix (thanks to new tests), and it saves 4.5kb now.

[behdad]

Saves 408 bytes building NotoSansArabic.

Now at 936 bytes after a few other fixes.