qt_gl_read_framebuffer does not work correctly.

QImage is initialized as:
QImage img(size, (alpha_format && include_alpha) ? QImage::Format_ARGB32_Premultiplied : QImage::Format_RGB32);

Since OpenGL does not support that premultiplied format, results are broken.

Workaround is to use:
QImage img(QSize(width, height), QImage::Format_RGBA8888);
GLint fmt = GL_RGBA;
glReadPixels(0, 0, w, h, fmt, GL_UNSIGNED_BYTE, img.bits());

Attached is the image how it looks like.
Left image is when scene is rendered using QOpenGLFramebufferObject and right image with the workaround used.