VarStore Format=1 is repeated in both attribute and element in ttx dump

[anthrotype]

I just noticed that when dumping an ItemVariationStore to ttx, the default Format=1 is written both in an attribute as well as as a sub-element:

fonttools/Tests/varLib/data/test_results/TestVariableCOLR-VF.ttx

Lines 185 to 186 in 9824263

	<VarStore Format="1">
	<Format value="1"/>

This is redundant, and I believe it's because in otData.py the VarStore is not defined as a format-switching table (there's in fact only one format right now). Changing it like that makes the redundant Format element go away, only the Format attribute stays:

diff --git a/Lib/fontTools/ttLib/tables/otData.py b/Lib/fontTools/ttLib/tables/otData.py
index 56716824e..1ae9a068e 100644
--- a/Lib/fontTools/ttLib/tables/otData.py
+++ b/Lib/fontTools/ttLib/tables/otData.py
@@ -3287,7 +3287,7 @@ otData = [
         ],
     ),
     (
-        "VarStore",
+        "VarStoreFormat1",
         [
             ("uint16", "Format", None, None, "Set to 1."),
             ("LOffset", "VarRegionList", None, None, ""),

But then a bunch of other tests fail when they import from existing XML files that do still contain a Format element, with KeyError like so:

    def getConverterByName(self, name):
>       return self.convertersByName[self.Format][name]
E       KeyError: 'Format'

Another way to fix it would be to specialise the toXML method of FormatSwitchingBaseTable, right now it uses BaseTable.toXML, which in turns has special handling of Format attribute (arguably that belongs to FormatSwitching variant only):

fonttools/Lib/fontTools/ttLib/tables/otBase.py

Lines 1117 to 1122 in 9824263

	def toXML(self, xmlWriter, font, attrs=None, name=None):
	tableName = name if name else self.__class__.__name__
	if attrs is None:
	attrs = []
	if hasattr(self, "Format"):
	attrs = attrs + [("Format", self.Format)]

Maybe there's no easy way to make this change backward compatible and we'll have to live with this duplication..

[behdad]

Ugh. Ugly. Let me think about it.