Change framebuffer.clear to use background color

samples/stm32f746g-discovery/disco-light.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:math';
 
 import 'package:stm32/stm32f746g_disco.dart';
 import 'package:stm32/lcd.dart';
 
 main() {
   // Initialize display, and get display dimensions.
   STM32F746GDiscovery discoBoard = new STM32F746GDiscovery();
   var display = discoBoard.frameBuffer;
   display.backgroundColor = Color.black;
   int w = display.width;
   int h = display.height;
 
   // Loop eternally drawing random lines.
   Random rnd = new Random(42);
   int lineCounter = 0;
   while (true) {
     // Render a line of random color from the centre to a random location.
     var color = new Color(rnd.nextInt(255), rnd.nextInt(255), rnd.nextInt(255));
     display.drawLine(w ~/ 2, h ~/ 2, rnd.nextInt(w), rnd.nextInt(h), color);
 
     // Every 100 lines, clear the display and update the counter.
     lineCounter += 1;
     if ((lineCounter % 100) == 0) {
-      display.clear(Color.black);
+      display.clear();
       display.writeText(10, 10, 'Rendered $lineCounter lines');
     }
   }
 }