title: the custom canvas in android view have an issue
body: <p>I define a custom canvas in a view,when running it will draw the background then draw some picture.and the background is an image too.When trigger <code>onTouchEvent</code>method ,will refresh the canvas,then draw background then few picutures,but there will leave the moving trace of later pictures on the background,I don't know how to clear the moving trace ,and if I assign a color to background,there will be no trace leave,Could anyone help me solve this?thank you very much.:D</p>
  
  <p>ADD:</p>
  
  <pre><code>canvas = new Canvas(bgbitmap);
  </code></pre>
  
  <p>draw background</p>
  
  <pre><code>canvas.drawColor(selectedColor);
  </code></pre>
  
  <p>if use drawBitmap draw background cause matter:</p>
  
  <pre><code>//                     canvas.drawBitmap(bgbitmap, 0, 0, null);
  
  // draw picutre
                      for (int i = fbmpCount - 1; i &gt;= 0; i--) {
                          FreeBitmap fb = findFreeBmpByPriority(i);
                          if (fb != null)
                              this.canvas.drawBitmap(fb.getBmp(), fb.getXpoint(), fb.getYpoint(),
                                      null);
                      }
  
          this.invalidate();
  
  // onDraw method
      @Override
      protected void onDraw(Canvas canvas) {
          super.onDraw(canvas);
          canvas.drawBitmap(bgbitmap, 0, 0, null);
      }
  </code></pre>