fix the order of prune_unused_regions

[qxliu76]

It should be after each vardata optimization.
Here's a testcase, I used this command fonttools varLib.instancer --no-overlap-flag --no-recalc-timestamp --no-optimize --output=ft.ttf test/subset/data/fonts/NotoSans-VF.abc.ttf wght=200:600 wdth=80:90 CTGR=20:60
File used is this: https://github.com/harfbuzz/harfbuzz/blob/main/test/subset/data/fonts/NotoSans-VF.abc.ttf
And generated MVAR table is like below:

<VarStore Format="1">
  <Format value="1"/>
  <VarRegionList>
    <!-- RegionAxisCount=3 -->
    <!-- RegionCount=4 -->
    <Region index="0">
  <!-- VarDataCount=1 -->
  <VarData index="0">
    <!-- ItemCount=2 -->
    <NumShorts value="0"/>
    <!-- VarRegionCount=2 -->
    <VarRegionIndex index="0" value="0"/>
    <VarRegionIndex index="1" value="3"/>
    <Item index="0" value="[-7, 6]"/>
    <Item index="1" value="[-4, 4]"/>
  </VarData>
</VarStore>

2 regions indexed at 1 and 2 are not used and not pruned.
This PR will fix it.

[anthrotype]

thanks, makes sense, not sure why it was done that way to begin with. We need to also update the test expectations. I can take care of it if you want

[qxliu76]

thanks, makes sense, not sure why it was done that way to begin with. We need to also update the test expectations. I can take care of it if you want

I have updated tests accordingly. Thanks!

[anthrotype]

Thanks 👍