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)

ctx.moveTo(x, y);
            if (width === 0 || height === 0) {
              ctx.lineTo(xw, yh);
            } else {
              ctx.lineTo(xw, y);
              ctx.lineTo(xw, yh);
              ctx.lineTo(x, yh);
            }
            ctx.closePath();
            break;
          case OPS.moveTo:
            x = args[j++];
            y = args[j++];
            ctx.moveTo(x, y);
            break;
          case OPS.lineTo:
            x = args[j++];
            y = args[j++];
            ctx.lineTo(x, y);
            break;
          case OPS.curveTo:
            x = args[j + 4];
            y = args[j + 5];
            ctx.bezierCurveTo(
              args[j],
              args[j + 1],
              args[j + 2],
              args[j + 3],
              x,
              y