Crash in minimal application's exit in XQueryExtension() after upgrade to Qt 6.2.1 from Qt 5.15.6 (on x11 but not wayland)

Just started playing with Qt 6...

But even my most trivial "minimal example" QQuickView-based application seems to Segmentation fault in the exit. gdb showed it to be in XQueryExtension().

Double checked same code works fine when built with 5.15.6.

Attached files are

• main.cpp, main.qml, resource.qrc - what you'd expect for an it-doesn't-get-simpler-than-this QML app.
• CMakeLists.txt - I'm new to CMake but this seems to work.
• MAKE-linux-native - captures the cmake invocation to build vs. Qt6.
• MAKE-linux-native-qt5 - captures the cmake invocation to build vs. Qt5.  NB Needs all the "Qt6" in CMakeLists.txt substituting with "Qt5" before using.

Stack trace for the crash-on-exit 6.2.1 build is
```

#0 0x00007ffff4bdef93 in XQueryExtension () at /usr/lib/x86_64-linux-gnu/libX11.so.6
 #1 0x00007ffff4bd2896 in XInitExtension () at /usr/lib/x86_64-linux-gnu/libX11.so.6
 #2 0x00007ffff24b90df in XextAddDisplay () at /usr/lib/x86_64-linux-gnu/libXext.so.6
 #3 0x00007ffff5229107 in () at /usr/lib/x86_64-linux-gnu/libGL.so.1
 #4 0x00007ffff522adda in () at /usr/lib/x86_64-linux-gnu/libGL.so.1
 #5 0x00007ffff522b5ca in () at /usr/lib/x86_64-linux-gnu/libGL.so.1
 #6 0x00007ffff5229a26 in () at /usr/lib/x86_64-linux-gnu/libGL.so.1
 #7 0x00007ffff522f1dc in () at /usr/lib/x86_64-linux-gnu/libGL.so.1
 #8 0x00007ffff52a5251 in () at /usr/lib/x86_64-linux-gnu/libGL.so.1
 #9 0x00007ffff7fe4715 in _dl_fini () at dl-fini.c:143
 #10 0x00007ffff5560d8c in __run_exit_handlers
 (status=0, listp=0x7ffff56e2718 <__exit_funcs>, run_list_atexit=run_list_atexit@entry=true, run_dtors=run_dtors@entry=true)
 at exit.c:108
 #11 0x00007ffff5560eba in __GI_exit (status=<optimized out>) at exit.c:139
 #12 0x00007ffff554b0a2 in __libc_start_main (main=
 0x555555555323 <main(int, char**)>, argc=1, argv=0x7fffffffe078, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7fffffffe068) at ../csu/libc-start.c:342
 #13 0x00005555555551aa in _start ()

```

While the build vs. 5.15.6 exits cleanly.

I don't have any Debian "bullseye" ("stable") machines currently but will start upgrading at some point and report back if that improves things.

 Update: Another data point... on another Debian buster/"olstable" amd64 machine with Intel's on-chip graphics (i7-770K) and Wayland graphics (XDG_SESSION_TYPE is "wayland"), the Qt6.2.1 build exits fine.  (FWIW, it also emits a message to the console "qt.qpa.wayland: Creating a fake screen in order for Qt not to crash").  Seems to imply this is something specific to x11 rather than wayland though.