code: function (symbol) {
          if (Array.isArray(symbol)) {
              for (var i = 0; i < symbol.length; i++) {
                  this.enableTexture(symbol[i]);
              }
              return;
          }
  
          var status = this._textureStatus[symbol];
          if (status) {
              var isEnabled = status.enabled;
              if (!isEnabled) {
                  status.enabled = true;
                  this._programKey = '';
              }
          }
      }
docs: Enable a texture, actually it will add a #define macro in the shader code
  For example, if texture symbol is diffuseMap, it will add a line `#define DIFFUSEMAP_ENABLED` in the shader code
  @param  {string} symbol