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)

colors[cIndex + 15] = colorsMap[cs[offset]];
              colors[cIndex + 16] = colorsMap[cs[offset] + 1];
              colors[cIndex + 17] = colorsMap[cs[offset] + 2];
              pIndex += 12;
              cIndex += 18;
            }
          }
          break;
        case "triangles":
          for (let j = 0, jj = ps.length; j < jj; j++) {
            coords[pIndex] = coordsMap[ps[j]];
            coords[pIndex + 1] = coordsMap[ps[j] + 1];
            colors[cIndex] = colorsMap[cs[j]];
            colors[cIndex + 1] = colorsMap[cs[j] + 1];
            colors[cIndex + 2] = colorsMap[cs[j] + 2];
            pIndex += 2;
            cIndex += 3;
          }
          break;
      }
    }
    
    if (backgroundColor) {
      gl.clearColor(
        backgroundColor[0] / 255,
        backgroundColor[1] / 255,
        backgroundColor[2] / 255,
        1.0
      );
    } else {