Clear the entire text buffer on the gpu when updating text lines

Author: einarf

demosys/effects/text/effects/writer_2d.py

@@ -50,6 +50,7 @@ def text_lines(self):
     @text_lines.setter
     def text_lines(self, value):
         self._text_lines = value
+        self._string_buffer.clear(chunk=b'\32')
 
         for i, line in enumerate(self._text_lines):
             self.set_text_line(i, line)