Screenshots/recordings should be affected by screen brightness

[runer112]

• [x] Screenshots
• [ ] APNG recordings

[adriweb]

Current implementation draws a black layer with alpha on top of the raw image, and only the raw image is used for screenshots and captures.

Potential solution: use the context painted images/bytes instead of the initial raw image?

[adriweb]

For screenshots, it's trivial, LCDWidget::getImage() should be:

    QPixmap pixmap(size());
    render(&pixmap);
    return pixmap.toImage();

instead of

    m_mutex.lock();
    QImage ret(m_image);
    m_mutex.unlock();
    return ret;

I'll commit that since it doesn't hurt (and also removes the mutex lock), but the problem is much harder if we want to fix that for APNG recordings, since apng_add_frame is called within LCDWidget::draw, and I don't think we want to (which may not be up to date at this point anyway, there is an event that's fired in it to update the widget later on)