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 = bytes.length;
    for (let i = 3; i < length; i += 4) {
      const alpha = bytes[i];
      if (alpha === 0) {
        bytes[i - 3] = r0;
        bytes[i - 2] = g0;
        bytes[i - 1] = b0;
      } else if (alpha < 255) {
        const alpha_ = 255 - alpha;
        bytes[i - 3] = (bytes[i - 3] * alpha + r0 * alpha_) >> 8;
        bytes[i - 2] = (bytes[i - 2] * alpha + g0 * alpha_) >> 8;
        bytes[i - 1] = (bytes[i - 1] * alpha + b0 * alpha_) >> 8;
      }
    }
  }
  function composeSMaskAlpha(maskData, layerData, transferMap) {
    const length = maskData.length;
    const scale = 1 / 255;
    for (let i = 3; i < length; i += 4) {
      const alpha = transferMap ? transferMap[maskData[i]] : maskData[i];
      layerData[i] = (layerData[i] * alpha * scale) | 0;
    }
  }
  function composeSMaskLuminosity(maskData, layerData, transferMap) {