add a stub/unused iGlobalTime uniform

glsl/raymarching.js

  
  var aPosition = gl.getAttribLocation(program, 'aPosition');
  var iResolution = gl.getUniformLocation(program, 'iResolution');
  var iGlobalTime = gl.getUniformLocation(program, 'iGlobalTime');
  var iMouse = gl.getUniformLocation(program, 'iMouse');
  
  gl.vertexAttrib2f(aPosition, 0.0, 0.0);
  gl.uniform2f(iResolution, canvas.width, canvas.height);
  gl.uniform1f(iGlobalTime, 0.0);
  gl.uniform3f(iMouse, 0.0, 0.0, 0.0);
  
  // two triangles