_DehintingT2Decompiler now inherits from T2WidthExtractor, so we can save
the charstring width and insert it back after we dropp the hints.
This avoids the need to do an extra .draw() just for the sake of
extracting the width.
We shall reuse it as base class for the _DehintingT2Decompiler in subset module,
as we need to save the width in order to re-insert it after dropping the hints.
This is the same patch as PR #606
Quoting myself:
```
It seems that the horizontal advance of hinted charstrings gets lost when the CFF hinting is stripped with '--no-hinting' option...
T2CharString objects only get assigned a 'witdh' attribute after their 'draw' method is called.
The subsetter's drop_hints function attempts to insert the width back at the beginning of the de-hinted charstring's program, but can do that only if the charstring does have a 'width' attribute:
c63fea0f8f/Lib/fontTools/subset/__init__.py (L1928)
Hence, we must 'draw' the charstring (with a NullPen) before stripping the hints.
```
Now this method fixes the issue (advances are kept) when doing _both_ --no-hinting and --desubroutinize (the test_no_hinting_desubroutinize should now pass, while it was failing before).
However, when one only does does --no-hinting, this method raises an error:
AttributeError: 'NoneType' object has no attribute 'nominalWidthX'
Lib/fontTools/misc/psCharStrings.py:282: AttributeError
(this is reflected in the failing `test_no_hinting_CFF`)
The following test cases are expected to fail because the current implementation of drop_hints
also strips the charstrings' advance widths:
Tests/subset/subset_test.py::SubsetTest::test_no_hinting_CFF
Tests/subset/subset_test.py::SubsetTest::test_no_hinting_desubroutinize_CFF
Indentation for TTX XMLF is 2 spaces. I chose 4 spaces for indentation of instructions to make it more noticeable. I hope it’s not a problem, because assembly code lines are usually very short.
Also fixes issues with merging of PairPos.
Trying on Noto Sans Thai still fails, now because of issues in class differences in
PairPosFormat2. :( Investigating.
This is part of fixing https://github.com/fonttools/fonttools/issues/719
though, the changes are currently in interpolate_layout, and need to be ported /
merged with varLib.__init__ variation-font-builder.
