Constant webxr::glwindow::SPHERICAL_VERTEX_SHADER

const SPHERICAL_VERTEX_SHADER: &[u8] = b"
  #version 330 core
  layout(location=0) in vec2 coord;
  out vec2 lon_lat;
  const float PI = 3.141592654;
  void main(void) {
    lon_lat = coord * vec2(PI, 0.5*PI);
    gl_Position = vec4(coord, 0.0, 1.0);
  }
";