otTables: add Paint.traverse method

paint graph traversal mostly adapted from rsheeter's https://github.com/googlefonts/nanoemoji/pull/233

Lib/fontTools/ttLib/tables/otTables.py

@@ -1367,6 +1367,40 @@ class Paint(getFormatSwitchingBaseTableClass("uint8")):
 		xmlWriter.endtag(tableName)
 		xmlWriter.newline()
 
+	def getChildren(self, colr):
+		if self.Format == PaintFormat.PaintColrLayers:
+			return colr.LayerV1List.Paint[
+				self.FirstLayerIndex : self.FirstLayerIndex + self.NumLayers
+			]
+
+		if self.Format == PaintFormat.PaintColrGlyph:
+			for record in colr.BaseGlyphV1List.BaseGlyphV1Record:
+				if record.BaseGlyph == self.Glyph:
+					return [record.Paint]
+			else:
+				raise KeyError(f"{self.Glyph!r} not in colr.BaseGlyphV1List")
+
+		children = []
+		for conv in self.getConverters():
+			if conv.tableClass is not None and issubclass(conv.tableClass, type(self)):
+				children.append(getattr(self, conv.name))
+
+		return children
+
+	def traverse(self, colr: COLR, callback):
+		"""Depth-first traversal of graph rooted at self, callback on each node."""
+		if not callable(callback):
+			raise TypeError("callback must be callable")
+		stack = [self]
+		visited = set()
+		while stack:
+			current = stack.pop()
+			if id(current) in visited:
+				continue
+			callback(current)
+			visited.add(id(current))
+			stack.extend(reversed(current.getChildren(colr)))
+
 
 # For each subtable format there is a class. However, we don't really distinguish
 # between "field name" and "format name": often these are the same. Yet there's