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 width = layer.width,
      height = layer.height;
    if (!smaskCache) {
      initSmaskGL();
    }
    const cache = smaskCache,
      canvas = cache.canvas,
      gl = cache.gl;
    canvas.width = width;
    canvas.height = height;
    gl.viewport(0, 0, gl.drawingBufferWidth, gl.drawingBufferHeight);
    gl.uniform2f(cache.resolutionLocation, width, height);
    if (properties.backdrop) {
      gl.uniform4f(
        cache.resolutionLocation,
        properties.backdrop[0],
        properties.backdrop[1],
        properties.backdrop[2],
        1
      );
    } else {
      gl.uniform4f(cache.resolutionLocation, 0, 0, 0, 0);
    }
    gl.uniform1i(
      cache.subtypeLocation,