I was wondering if WebGL is considered stable enough to be enabled by default. It makes some PDFs render much faster and it has been off by default for a while now, so we have had quite some time to test it.

If we decide to enable WebGL by default, we should not forget to update https://github.com/mozilla/pdf.js/blob/master/extensions/chromium/preferences_schema.json#L39 as well.

/cc @yurydelendik 
/cc @Snuffleupagus (because IIRC you were testing WebGL too)

case "BM":
            this.ctx.globalCompositeOperation = value;
            break;
          case "SMask":
            if (this.current.activeSMask) {
              
              
              
              if (
                this.stateStack.length > 0 &&
                this.stateStack[this.stateStack.length - 1].activeSMask ===
                  this.current.activeSMask
              ) {
                this.suspendSMaskGroup();
              } else {
                this.endSMaskGroup();
              }
            }
            this.current.activeSMask = value ? this.tempSMask : null;
            if (this.current.activeSMask) {
              this.beginSMaskGroup();
            }
            this.tempSMask = null;
            break;
          case "TR":
            this.current.transferMaps = value;
        }
      }
    }
    beginSMaskGroup() {