code: function WebGlEquirectRenderer(gl) {
    this.gl = gl;
  
    // The inverse projection matrix.
    this.invProjMatrix = mat4.create();
  
    // The viewport matrix responsible for viewport clamping.
    // See setViewport() for an explanation of how it works.
    this.viewportMatrix = mat4.create();
  
    this.constantBuffers = createConstantBuffers(gl, vertexIndices, vertexPositions, textureCoords);
  
    this.shaderProgram = createShaderProgram(gl, vertexSrc, fragmentSrc, attribList, uniformList);
  }
docs: @class WebGlEquirectRenderer
  @implements Renderer
  @classdesc
  
  A renderer for {@link EquirectGeometry} and {@link RectilinearView},
  appropriate for {@link WebGlStage}.
  
  Most users do not need to instantiate this class. Renderers are created and
  destroyed by {@link Stage} as necessary.