add ClipList.Format so we can bump when we finally get >16bit GIDs

https://github.com/googlefonts/colr-gradients-spec/issues/346

Lib/fontTools/colorLib/builder.py

@@ -252,6 +252,7 @@ def buildCOLR(
 
 def buildClipList(clipBoxes: Dict[str, _ClipBoxInput]) -> ot.ClipList:
     clipList = ot.ClipList()
+    clipList.Format = 0
     clipList.clips = {name: buildClipBox(box) for name, box in clipBoxes.items()}
     return clipList
 

Lib/fontTools/ttLib/tables/otData.py

@@ -1601,7 +1601,8 @@ otData = [
 		('LOffset', 'Paint', 'LayerCount', 0, 'Array of offsets to Paint tables, from the start of the LayerList table.'),
 	]),
 
-	('ClipList', [
+	('ClipListFormat0', [
+		('uint8', 'Format', None, None, 'Format for ClipList with 16bit glyph IDs: 0'),
 		('uint32', 'ClipCount', None, None, 'Number of Clip records.'),
 		('struct', 'ClipRecord', 'ClipCount', 0, 'Array of Clip records sorted by glyph ID.'),
 	]),

Lib/fontTools/ttLib/tables/otTables.py

@@ -1307,7 +1307,7 @@ class ClipBox(getFormatSwitchingBaseTableClass("uint8")):
 		return f"{self.__class__.__name__}{self.as_tuple()}"
 
 
-class ClipList(BaseTable):
+class ClipList(getFormatSwitchingBaseTableClass("uint8")):
 
 	def populateDefaults(self, propagator=None):
 		if not hasattr(self, "clips"):
@@ -1410,6 +1410,8 @@ class ClipList(BaseTable):
 		tableName = name if name else self.__class__.__name__
 		if attrs is None:
 			attrs = []
+		if hasattr(self, "Format"):
+			attrs.append(("Format", self.Format))
 		xmlWriter.begintag(tableName, attrs)
 		xmlWriter.newline()
 		# sort clips alphabetically to ensure deterministic XML dump

Tests/colorLib/builder_test.py

@@ -1681,6 +1681,7 @@ class BuildCOLRTest(object):
             },
         )
 
+        assert colr.table.ClipList.Format == 0
         clipBoxes = colr.table.ClipList.clips
         assert [
             (baseGlyph, clipBox.as_tuple()) for baseGlyph, clipBox in clipBoxes.items()

Tests/ttLib/tables/C_O_L_R_test.py

@@ -298,13 +298,14 @@ COLR_V1_SAMPLE = (
     (b" \x00", "Paint.Alpha (0.5)"),
 
     # ClipList
+    (b'\x00', "ClipList.Format (0)"),
     (b'\x00\x00\x00\x02', "ClipList.ClipCount (2)"),
     (b'\x00\x0a', "ClipRecord[0].StartGlyphID (10)"),
     (b'\x00\x0a', "ClipRecord[0].EndGlyphID (10)"),
-    (b'\x00\x00\x12', "Offset to ClipBox subtable from beginning of ClipList (18)"),
+    (b'\x00\x00\x13', "Offset to ClipBox subtable from beginning of ClipList (19)"),
     (b'\x00\x0e', "ClipRecord[1].StartGlyphID (14)"),
     (b'\x00\x0f', "ClipRecord[1].EndGlyphID (15)"),
-    (b'\x00\x00\x1f', "Offset to ClipBox subtable from beginning of ClipList (31)"),
+    (b'\x00\x00\x20', "Offset to ClipBox subtable from beginning of ClipList (32)"),
 
     (b'\x01', "ClipBox.Format (1)"),
     (b'\x00\x00', "ClipBox.xMin (0)"),
@@ -505,7 +506,7 @@ COLR_V1_XML = [
     '    <dy value="258"/>',
     "  </Paint>",
     "</LayerList>",
-    "<ClipList>",
+    '<ClipList Format="0">',
     "  <Clip>",
     '    <Glyph value="glyph00010"/>',
     '    <ClipBox Format="1">',