Change framebuffer.clear to use background color

pkg/stm32/lib/lcd.dart

Index: pkg/stm32/lib/lcd.dart
diff --git a/pkg/stm32/lib/lcd.dart b/pkg/stm32/lib/lcd.dart
index 9d1f84d7654fb9bb7afdabff4edf2c4bf71ff6e1..0dc6fc76d08f49b3633f0b363bb7057a1afa8ee8 100644
--- a/pkg/stm32/lib/lcd.dart
+++ b/pkg/stm32/lib/lcd.dart
@@ -81,18 +81,22 @@ class FrameBuffer {
   int get height => _lcdHeight.icall$0();
   int get width => _lcdWidth.icall$0();
 
-  clear([Color color = Color.black]) {
-    _lcdClear.icall$1(color.rgb8888);
-  }
+  Color _backgroundColor = Color.white;
 
-  set foregroundColor(Color color) {
-    _lcdSetForegroundColor.icall$1(color.rgb8888);
+  clear([Color color]) {
+    color ??= _backgroundColor;
+    _lcdClear.icall$1(color.rgb8888);
   }
 
   set backgroundColor(Color color) {
+    _backgroundColor = color;
     _lcdSetBackgroundColor.icall$1(color.rgb8888);
   }
 
+  set foregroundColor(Color color) {
+    _lcdSetForegroundColor.icall$1(color.rgb8888);
+  }
+
   Color readPixel(int x, int y) {
     return new Color.fromRgb8888(_lcdReadPixel.icall$2(x, y));
   }