function init(gl) {
      if (!enums) {
        enums = {};
        Object.keys(gl).forEach(function(key) {
          if (typeof gl[key] === 'number') {
            enums[gl[key]] = key;
          }
        });
      }
    }
Gets a string for gl enum

Note: Several enums are the same. Without more
context (which function) it's impossible to always
give the correct enum.

@param {WebGLRenderingContext} gl A WebGLRenderingContext
@param {number} value the value of the enum you want to look up.