Specs: make sure all layout/paint/hitTest APIs are relative to the right coordinate space

sky/examples/style/toolbar-layout.sky

 SKY MODULE
 <import src="sky:core" as="sky"/>
 <script>
  // display: toolbar;
  // toolbar-spacing: <length>
  // display: spring; // remaining space is split equally amongst the springs
  // children are vertically centered, layout out left-to-right with toolbar-spacing space between them
  // last child is hidden by default unless there's not enough room for the others, then it's shown last, right-aligned
  module.exports.SpringLayoutManager = class SpringLayoutManager extends sky.LayoutManager { }
  sky.registerLayoutManager('spring', module.exports.SpringLayoutManager);
@@ skipping 177 matching lines @@
      while ((!loop.done) && (loop.value != this.firstSkippedChild))
        this.paintChild(loop.value, canvas);
      if (this.showingOverflow)
        this.paintChild(this.overflowChild, canvas);
    }
    function paintChild(child, canvas) {
      if (child.needsPaint) {
        canvas.save();
        try {
          canvas.beginPath();
-         canvas.rect(child.x, child.y, child.width, child.height);
+         canvas.translate(child.x, child.y);
+         canvas.rect(0, 0, child.width, child.height);
          canvas.clip();
          child.paint(canvas);
        } finally {
          canvas.restore();
        }
      }
    }
  }
  sky.registerLayoutManager('toolbar', module.exports.ToolbarLayoutManager);
 </script>