title: No useful QOpenGLFramebufferObject::blitFramebuffer() and texture() when it encounters multisampling
body: <p>I am doiong some offscreen rendering job and I need to do multisampling and some blur job,  so I created FBO_A to do my drawing job, then I use the blitFramebuffer() like <a href="https://www.qt.io/blog/2015/09/21/using-modern-opengl-es-features-with-qopenglframebufferobject-in-qt-5-6" rel="nofollow noreferrer">what Qt blog did</a>   to blit what in FBO_A to FBO_B whose sample was set to 16, but what I got in FBO_B (use QOpenGLFramebufferObject::toImage) is still a no multisampling job. I have to say it is strange because when I set FBO_A sample to 16 and draw directly, then I got the multisampling image, which is totally different from what the above article said(You have to use blit to get a downsample  or multisample FBO).</p>
  <p>Another question is about copy multisampling texture. I created FBO_C and want to draw FBO_A's content as a  texture, the strange thing happened here, when FBO_A is a no multisampling fbo, I cant use texture() get the handle and FBO_C get the correct answer. But when FBO_A is set to multisampling, then the texture() get the right handle too, but u can never draw it in FBO_C correctly, what u get is always a black image.</p>
  <p>My Qt is 5.9.9,  MSVC 2015 32bit.</p>