add dot grid background

lib/core/background.dart

@@ -9,24 +9,50 @@ abstract class CanvasBackground {
 
   /// draw the backround on this context. Implement this to have
   /// different kinds of backgrounds
-  void paint(Canvas canvas, Size canvasSize, double scale);
+  void paint(Canvas canvas, Offset offset, double scale, Size canvasSize);
 }
 
 class NoBackground extends CanvasBackground {
   const NoBackground();
 
   @override
-  void paint(Canvas canvas, Size canvasSize, double scale) {}
+  void paint(Canvas canvas, Offset offset, double scale, Size canvasSize) {}
 }
 
 class SingleColorBackround extends CanvasBackground {
   const SingleColorBackround(Color backgroundColor) : super(bgColor: backgroundColor);
 
   @override
-  void paint(Canvas canvas, Size canvasSize, double scale) {
+  void paint(Canvas canvas, Offset offset, double scale, Size canvasSize) {
     final paint = Paint()
       ..color = bgColor
       ..style = PaintingStyle.fill;
     canvas.drawRect(Rect.fromLTWH(0, 0, canvasSize.width, canvasSize.height), paint);
   }
 }
+
+class DotGridBackround extends CanvasBackground {
+  final double size;
+  final double spacing;
+  final Color color;
+
+  const DotGridBackround({this.size = 2.0, this.spacing = 50.0, this.color = Colors.black12}) : super();
+
+  @override
+  void paint(Canvas canvas, Offset offset, double scale, Size canvasSize) {
+    final paint = Paint()
+      ..color = color
+      ..strokeWidth = 1.0 * scale;
+    double scaledSpacing = spacing * scale;
+    double scaledSize = size * scale;
+
+    double startX = (offset.dx % scaledSpacing) - scaledSpacing;
+    double startY = (offset.dy % scaledSpacing) - scaledSpacing;
+
+    for (double x = startX; x < canvasSize.width + scaledSpacing; x += scaledSpacing) {
+      for (double y = startY; y < canvasSize.height + scaledSpacing; y += scaledSpacing) {
+        canvas.drawCircle(Offset(x, y), scaledSize, paint);
+      }
+    }
+  }
+}

lib/core/render.dart

@@ -216,7 +216,7 @@ class _CanvasRenderBox extends RenderBox
   void paint(PaintingContext context, Offset canvasStartOffset) {
     assert(childCount == _ssPositions.length);
     // use the canvas background painter, pass it the canvas and that should handle drawing the background
-    _canvasBackground.paint(context.canvas, size, _scale);
+    _canvasBackground.paint(context.canvas, canvasStartOffset, _scale, size);
 
     // though using ssPositionns here directly worked for me but docs using the parentData
     // to get this info is the convention as child can be reordered ( though this will always