Bug 1958121 - Use exchange to update the SpaceFeatures flags. a=RyanVM

void gfxFont::CheckForFeaturesInvolvingSpace() const {

  gfxFontEntry::SpaceFeatures flags = gfxFontEntry::SpaceFeatures::None;

  // mFontEntry->mHasSpaceFeatures is a std::atomic<>, so we set it with

  // `exchange` to avoid a potential data race. It's ok if two threads both

  // try to set it; they'll end up with the same value, so it doesn't matter

  // that one will overwrite the other.

  auto setFlags =

      MakeScopeExit([&]() { mFontEntry->mHasSpaceFeatures = flags; });

      MakeScopeExit([&]() { mFontEntry->mHasSpaceFeatures.exchange(flags); });

  bool log = LOG_FONTINIT_ENABLED();

  TimeStamp start;