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.strokeStyle = "#000000";
    ctx.fillStyle = "#000000";
    ctx.fillRule = "nonzero";
    ctx.globalAlpha = 1;
    ctx.lineWidth = 1;
    ctx.lineCap = "butt";
    ctx.lineJoin = "miter";
    ctx.miterLimit = 10;
    ctx.globalCompositeOperation = "source-over";
    ctx.font = "10px sans-serif";
    if (ctx.setLineDash !== undefined) {
      ctx.setLineDash([]);
      ctx.lineDashOffset = 0;
    }
  }
  function composeSMaskBackdrop(bytes, r0, g0, b0) {