Add overdraw visualization to the debugger

https://codereview.appspot.com/7267043/



git-svn-id: http://skia.googlecode.com/svn/trunk@7627 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/debugger/QT/SkRasterWidget.cpp b/debugger/QT/SkRasterWidget.cpp
index a8a36a8..db3d00f 100644
--- a/debugger/QT/SkRasterWidget.cpp
+++ b/debugger/QT/SkRasterWidget.cpp
@@ -27,6 +27,7 @@
 void SkRasterWidget::resizeEvent(QResizeEvent* event) {
     fBitmap.setConfig(SkBitmap::kARGB_8888_Config, event->size().width(), event->size().height());
     fBitmap.allocPixels();
+    fBitmap.eraseColor(SK_ColorTRANSPARENT);
     SkSafeUnref(fCanvas);
     SkSafeUnref(fDevice);
     fDevice = new SkDevice(fBitmap);