[AAT] Do not assume that StructWithLength is always 32 bits long

Most AAT tables encode struct lengths into 32 bits but some use only 16.
Therefore, take the size of the encoded struct length from otData.

Lib/fontTools/ttLib/tables/otConverters.py

@@ -447,22 +447,24 @@ class StructWithLength(Struct):
 		table = self.tableClass()
 		table.decompile(reader, font)
 		reader.seek(pos + table.StructLength)
-		del table.StructLength
 		return table
 
 	def write(self, writer, font, tableDict, value, repeatIndex=None):
-		value.StructLength = 0xdeadbeef
+		for convIndex, conv in enumerate(value.getConverters()):
+			if conv.name == "StructLength":
+				break
+		lengthIndex = len(writer.items) + convIndex
+		deadbeef = {1:0xDE, 2:0xDEAD, 4:0xDEADBEEF}[conv.staticSize]
+
 		before = writer.getDataLength()
-		i = len(writer.items)
+		value.StructLength = deadbeef
 		value.compile(writer, font)
 		length = writer.getDataLength() - before
-		for j,conv in enumerate(value.getConverters()):
-			if conv.name != 'StructLength':
-				continue
-			assert writer.items[i+j] == b"\xde\xad\xbe\xef"
-			writer.items[i+j] = struct.pack(">L", length)
-			break
-		del value.StructLength
+		lengthWriter = writer.getSubWriter()
+		conv.write(lengthWriter, font, tableDict, length)
+		assert(writer.items[lengthIndex] ==
+		       b"\xde\xad\xbe\xef"[:conv.staticSize])
+		writer.items[lengthIndex] = lengthWriter.getAllData()
 
 
 class Table(Struct):