This also fixes fonttools/fonttools/issues/1030.
Although the roundtrip is generally possible when a VariationStore is built from source font data using the Superpolator model, it is possible to build region definitions that do not follow this model. Behdad cited the Skia "Q" example, where the tail of the Q is affected by two regions defined as:
min=0 peak=0.5 max=0.51 delta=+10
min=0.49 peak=0.5 max=0.51 delta=-10
We don't need to cast to int when using the round function from py23,
as this is a backport of python3's built-in round and thus it returns
an int when called with a single argument.
* Removed `CFFContext`
* Added `isCFF2` argument to CFFFontSet.decompile/compile, used from
respective ttLib classes
* Index classes get a `isCFF2` argument in constructor (used for
decompiling); must be True/False if `file` argument is not None;
it is stored as self._isCFF2 to support lazy loading
* Removed `TopDictData` class; reuse same `TopDictIndexCompiler` for
both CFF and CFF2
* `CFFWriter` and all `*Compiler` classes get an `isCFF2` argument;
defaults to the parent compiler's `isCFF2` attribute
* Removed `size` argument from `produceItem` method as unused and
useless (`len(data)` is the same)
* psCharStrings: removed useless ByteCodeBase class
* A reference to the TopDict's VarStoreData is passed down to all
the FontDicts' PrivateDict, so it can be used to get the number of
regions while decompiling blend and vsindex operators
See dicussion:
https://github.com/fonttools/fonttools/pull/968#issuecomment-309920007
SimpleCFF2DEcompiler
CFF2CharString
GlobalSubrsIndex2
SubrsIndex2
CharstringIndex2
Working towards using one set of classes for both CFF2 and CFF data.
After 2b2aca1, DictCompiler/Decompiler's `arg_delta` method unconditionally attempts to get the first item to check if it's a list, but this fails with `IndexError` when the value is empty.
```
Traceback (most recent call last):
[...]
File "/Users/cosimolupo/Documents/Github/ufo2ft/Lib/ufo2ft/otfPostProcessor.py", line 15, in __init__
otf.save(stream)
File "/Users/cosimolupo/Documents/Github/fonttools/Lib/fontTools/ttLib/__init__.py", line 219, in save
self._writeTable(tag, writer, done)
File "/Users/cosimolupo/Documents/Github/fonttools/Lib/fontTools/ttLib/__init__.py", line 658, in _writeTable
tabledata = self.getTableData(tag)
File "/Users/cosimolupo/Documents/Github/fonttools/Lib/fontTools/ttLib/__init__.py", line 669, in getTableData
return self.tables[tag].compile(self)
File "/Users/cosimolupo/Documents/Github/fonttools/Lib/fontTools/ttLib/tables/C_F_F_.py", line 20, in compile
self.cff.compile(f, otFont)
File "/Users/cosimolupo/Documents/Github/fonttools/Lib/fontTools/cffLib.py", line 124, in compile
writer.toFile(file)
File "/Users/cosimolupo/Documents/Github/fonttools/Lib/fontTools/cffLib.py", line 300, in toFile
endPos = pos + item.getDataLength()
File "/Users/cosimolupo/Documents/Github/fonttools/Lib/fontTools/cffLib.py", line 1858, in getDataLength
return len(self.compile("getDataLength"))
File "/Users/cosimolupo/Documents/Github/fonttools/Lib/fontTools/cffLib.py", line 1879, in compile
data.append(arghandler(value))
File "/Users/cosimolupo/Documents/Github/fonttools/Lib/fontTools/cffLib.py", line 1910, in arg_delta
val0 = value[0]
IndexError: list index out of range
``
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.
