Change framebuffer.clear to use background color

samples/stm32f746g-discovery/turtle-graphics/turtle.dart

Index: samples/stm32f746g-discovery/turtle-graphics/turtle.dart
diff --git a/samples/stm32f746g-discovery/turtle-graphics/turtle.dart b/samples/stm32f746g-discovery/turtle-graphics/turtle.dart
index d18fcd86d4e7447ecc8054ec67b9ef065f419f22..71feb98bf9e34926511d3f0f17d140037cf377f3 100644
--- a/samples/stm32f746g-discovery/turtle-graphics/turtle.dart
+++ b/samples/stm32f746g-discovery/turtle-graphics/turtle.dart
@@ -20,6 +20,7 @@ class Turtle {
 
   /// Create a new Turtle, and set initial location.
   Turtle(this._display, {xPosition: 0, yPosition: 0}) {
+    _display.backgroundColor = Color.black;
     _display.clear();
     _x = xPosition;
     _y = yPosition;