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)

this._transformMatrix = old.slice(0, 6);
    this._originalSave();
  };
  ctx.restore = function ctxRestore() {
    const prev = this._transformStack.pop();
    if (prev) {
      this._transformMatrix = prev;
      this._originalRestore();
    }
  };
  ctx.translate = function ctxTranslate(x, y) {
    const m = this._transformMatrix;
    m[4] = m[0] * x + m[2] * y + m[4];
    m[5] = m[1] * x + m[3] * y + m[5];
    this._originalTranslate(x, y);
  };
  ctx.scale = function ctxScale(x, y) {
    const m = this._transformMatrix;
    m[0] = m[0] * x;
    m[1] = m[1] * x;
    m[2] = m[2] * y;
    m[3] = m[3] * y;
    this._originalScale(x, y);
  };
  ctx.transform = function ctxTransform(a, b, c, d, e, f) {
    const m = this._transformMatrix;
    this._transformMatrix = [
      m[0] * a + m[2] * b,
      m[1] * a + m[3] * b,
      m[0] * c + m[2] * d,