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)

for (let row = 1; row < rows; row++) {
            let offset = row * cols + 1;
            for (let col = 1; col < cols; col++, offset++) {
              coords[pIndex] = coordsMap[ps[offset - cols - 1]];
              coords[pIndex + 1] = coordsMap[ps[offset - cols - 1] + 1];
              coords[pIndex + 2] = coordsMap[ps[offset - cols]];
              coords[pIndex + 3] = coordsMap[ps[offset - cols] + 1];
              coords[pIndex + 4] = coordsMap[ps[offset - 1]];
              coords[pIndex + 5] = coordsMap[ps[offset - 1] + 1];
              colors[cIndex] = colorsMap[cs[offset - cols - 1]];
              colors[cIndex + 1] = colorsMap[cs[offset - cols - 1] + 1];
              colors[cIndex + 2] = colorsMap[cs[offset - cols - 1] + 2];
              colors[cIndex + 3] = colorsMap[cs[offset - cols]];
              colors[cIndex + 4] = colorsMap[cs[offset - cols] + 1];
              colors[cIndex + 5] = colorsMap[cs[offset - cols] + 2];
              colors[cIndex + 6] = colorsMap[cs[offset - 1]];
              colors[cIndex + 7] = colorsMap[cs[offset - 1] + 1];
              colors[cIndex + 8] = colorsMap[cs[offset - 1] + 2];
              coords[pIndex + 6] = coords[pIndex + 2];
              coords[pIndex + 7] = coords[pIndex + 3];
              coords[pIndex + 8] = coords[pIndex + 4];
              coords[pIndex + 9] = coords[pIndex + 5];
              coords[pIndex + 10] = coordsMap[ps[offset]];
              coords[pIndex + 11] = coordsMap[ps[offset] + 1];
              colors[cIndex + 9] = colors[cIndex + 3];
              colors[cIndex + 10] = colors[cIndex + 4];
              colors[cIndex + 11] = colors[cIndex + 5];
              colors[cIndex + 12] = colors[cIndex + 6];
              colors[cIndex + 13] = colors[cIndex + 7];
              colors[cIndex + 14] = colors[cIndex + 8];