gtkglext and Qt don't work fine togather

[Fabounet]

Hi,
somewhere gtkglext sets a custom colormap on the root window. This causes all Qt apps which have embeded video (Smplayer, Skype, VLC, etc) to be invisible.

This happens when we want to get an opengl config with an alpha channel (ARGB window).
It seems to happen during the call to gdk_x11_gl_config_new_from_visualid()
the visual is got by glXGetVisualFromFBConfig(), and gtkgl is used with no specific option. The problem occurs on any distro and any WM.

after that function, a call to "xprop -root" gives:
> RGB_DEFAULT_MAP(RGB_COLOR_MAP):
> colormap id #: 0x1400001
> red-max: 255
> red-mult: 65536
> green-max: 255
> green-mult: 256
> blue-max: 255
> blue-mult: 1
> base-pixel: 0
> visual id #: 0x8c
calling "xprop -root -remove RGB_DEFAULT_MAP" or setting XLIB_SKIP_ARGB_VISUALS allow Smplayer/Skype to work correctly.

here is the complete story: https://bugs.kde.org/show_bug.cgi?id=262064

A way to reproduce it is to launch Cairo-Dock with the opengl backend.
I didn't see anything in the code that would set a colormap, so the bug is maybe in GDK.