docs: function SpriteMaskFilter(sprite)
  {
      var maskMatrix = new math.Matrix();
  
      AbstractFilter.call(this,
          fs.readFileSync(__dirname + '/spriteMaskFilter.vert', 'utf8'),
          fs.readFileSync(__dirname + '/spriteMaskFilter.frag', 'utf8'),
          {
              mask:           { type: 'sampler2D', value: sprite._texture },
              alpha:          { type: 'f', value: 1},
              otherMatrix:    { type: 'mat3', value: maskMatrix.toArray(true) }
          }
      );
  
      this.maskSprite = sprite;
      this.maskMatrix = maskMatrix;
  }