anchor.diff:  }
   
   class BackgroundDotRenderingSystem extends WebGlRenderingSystem {
  +  TagManager tm;
     Mapper<Position> pm;
     Mapper<Color> cm;
     Mapper<Background> bm;
  
positive.code: 
    @override
    String get vShaderSource => '''
  attribute vec2 a_Position;
  attribute vec4 a_Color;
  varying vec4 v_Color;
  void main() {
    gl_Position = vec4(a_Position.x / 400.0 - 1.0, -(a_Position.y / 300.0 - 1.0), 0.0, 1.0);
    gl_PointSize = 1.0;
    v_Color = a_Color;
  }
  
negative.code: 
    @override
    String get vShaderSource => '''
  uniform mat4 uModelMatrix;
  attribute float a_Size;
  attribute vec2 a_Position;
  attribute vec4 a_Color;
  varying vec4 v_Color;
  void main() {
    gl_Position = uModelMatrix * vec4(a_Position.x / 400.0 - 1.0, -(a_Position.y / 300.0 - 1.0), 0.0, 1.0);
    gl_PointSize = a_Size;
    v_Color = a_Color;
  }
  