Change framebuffer.clear to use background color

samples/stm32f746g-discovery/touch.dart

 // Copyright (c) 2016, the Dartino project authors. Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE.md file.
 
 import 'dart:dartino';
 
 import 'package:stm32/lcd.dart';
 import 'package:stm32/stm32f746g_disco.dart';
 import 'package:stm32/ts.dart';
 
 main() {
   var disco = new STM32F746GDiscovery();
   var frameBuffer = disco.frameBuffer;
   var touchScreen = disco.touchScreen;
 
   frameBuffer.backgroundColor = Color.white;
-  frameBuffer.clear(Color.white);
+  frameBuffer.clear();
   int x = 25;
   int y = 25;
   frameBuffer.drawLine(x, y - 5, x, y + 5, Color.blue);
   frameBuffer.drawLine(x - 5, y, x + 5, y, Color.blue);
   frameBuffer.writeText(x + 3, y + 3, "$x, $y");
   x = frameBuffer.width - 25;
   frameBuffer.drawLine(x, y - 5, x, y + 5, Color.blue);
   frameBuffer.drawLine(x - 5, y, x + 5, y, Color.blue);
   frameBuffer.writeText(x - 50, y + 3, "$x, $y");
 
   while (true) {
     TouchState t = touchScreen.state;
     var msg = new StringBuffer('touch: ${t.count}');
     for (int index = 0; index < t.count; ++index) {
       msg.write(' - ${t.x[index]}, ${t.y[index]}');
     }
     print(msg);
     sleep(500);
   }
 }