Create subReaders lazily

Should improve memory usage
diff --git a/Lib/fontTools/ttLib/tables/otBase.py b/Lib/fontTools/ttLib/tables/otBase.py
index 4fcc411..f3876f4 100644
--- a/Lib/fontTools/ttLib/tables/otBase.py
+++ b/Lib/fontTools/ttLib/tables/otBase.py
@@ -585,8 +585,9 @@
 	def ensureDecompiled(self):
 		if self.compileStatus != 1:
 			return
-		self.decompile(self.reader, self.font)
-		del self.reader, self.font
+		subReader = self.reader.getSubReader(self.offset)
+		self.decompile(subReader, self.font)
+		del self.reader, self.font, self.offset
 
 	def preCompile(self):
 		pass # used only by the LookupList class