Describe the bug
When using gl_FragDepthEXT in a WebGL 1 context with exokit, the following error message is printed on the command line even when GL_EXT_frag_depth is enabled (unimportant parts omitted in the text below).

[
  '#extension GL_EXT_frag_depth : enable',
  'precision highp float;',
  'uniform vec3 uAmbient;',
  ...
  'gl_FragDepthEXT = (projPoint.z / projPoint.w + 1.0) / 2.0;',
  ...
]
v {
  message: 'Error compiling shader:\n0(158) : error C7531: global variable ' +
    'gl_FragDepthEXT requires "#extension GL_EXT_frag_depth : enable" ' +
    'before use\n'
}

To Reproduce
Open any web page using GL_EXT_frag_depth and gl_FragDepthEXT in exokit.

System information:

• OS: Ubuntu 18.04

• Exokit Version: master branch from 2019-08-27 (today)

• Node version: v12.2.0

• Is your Exokit downloaded or built? Built

• GPU: NVIDIA GTX 1070

• Driver version: 430.40