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)

const length = maskData.length;
    for (let i = 3; i < length; i += 4) {
      const y =
        maskData[i - 3] * 77 + 
        maskData[i - 2] * 152 + 
        maskData[i - 1] * 28; 
      layerData[i] = transferMap
        ? (layerData[i] * transferMap[y >> 8]) >> 8
        : (layerData[i] * y) >> 16;
    }
  }
  function genericComposeSMask(
    maskCtx,
    layerCtx,
    width,
    height,
    subtype,
    backdrop,
    transferMap
  ) {
    const hasBackdrop = !!backdrop;
    const r0 = hasBackdrop ? backdrop[0] : 0;
    const g0 = hasBackdrop ? backdrop[1] : 0;
    const b0 = hasBackdrop ? backdrop[2] : 0;
    let composeFn;
    if (subtype === "Luminosity") {