Black screen in QtOpenGL apps under Xvnc4 when using Mesa with Gallium or XLib drivers

Used hellogl2 sample to reproduce the issue. Built it from source in QtCreator.

Here are the steps:

1. Launch the sample in the default desktop: it works.

2. Start VNC server with: vnc4server and see which display it occupies

3. Launch DISPLAY=:<number-of-vnc-display> hellogl2

Result: there are errors like that

qt.qpa.xcb: failed to initialize XRandr
qt.qpa.xcb: XKeyboard extension not present on the X server
qt.glx: qglx_findConfig: Failed to finding matching FBConfig for QSurfaceFormat(version 2.0, options QFlags<QSurfaceFormat::FormatOption>(), depthBufferSize 0, redBufferSize 1, greenBufferSize 1, blueBufferSize 1, alphaBufferSize -1, stencilBufferSize -1, samples -1, swapBehavior QSurfaceFormat::SingleBuffer, swapInterval 1, colorSpace QSurfaceFormat::DefaultColorSpace, profile QSurfaceFormat::NoProfile)
Unsupported screen format: depth: 16, bits_per_pixel: 16, red_mask: 3f, blue_mask: f800
Using RGB16 fallback, if this works your X11 server is reporting a bad screen format.

and the app is black.

4. Get Mesa libGL.so* with Gallium or Xlib driver and put it into any directory

5. Launch DISPLAY=:<number-of-vnc-display> LD_LIBRARY_PATH=<path-to-mesa-dir> hellogl2

Result:

There is still an error but it looks not so severe:

qt.qpa.xcb: failed to initialize XRandr
qt.qpa.xcb: XKeyboard extension not present on the X server

Result: 

Buttons and sliders appeared but the OpenGL widget is still black.

6. Launch: DISPLAY=:<number-of-vnc-display> LD_LIBRARY_PATH=<path-to-mesa-dir> glxgears

Result: it works.