Qt Example/Tests trigger VTK error about OpenGL

[mwestphal]

Describe the bug
Our Qt examples trigger error about OpenGL within VTK, which is unexpected and may be a symptom of an issue to fix

To Reproduce
Steps to reproduce the behavior:

1. Patch to display VTK errors:

diff --git a/library/testing/TestSDKExternalWindowQT.cxx b/library/testing/TestSDKExternalWindowQT.cxx
index f42b5bb0..0ab23999 100644
--- a/library/testing/TestSDKExternalWindowQT.cxx
+++ b/library/testing/TestSDKExternalWindowQT.cxx
@@ -81,6 +81,7 @@ private:
 
 int TestSDKExternalWindowQT([[maybe_unused]] int argc, [[maybe_unused]] char* argv[])
 {
+  f3d::log::setVerboseLevel(f3d::log::VerboseLevel::DEBUG);
   QGuiApplication a(argc, argv);
 
   F3DWindow w(

2. Build with BUILD_TESTING and F3D_TESTING_ENABLE_EXTERNAL_GL
3. ctest -R TestSDKExternalWindowQT -VV
4. Errors in log:

57: 
57: ERROR: In vtkGenericOpenGLRenderWindow.cxx, line 203
57: vtkF3DExternalRenderWindow (0x55a4f5b9a910): error before running VTK rendering code 1 OpenGL errors detected
57:   0 : (1282) Invalid operation
57: 

There is no issues apart the error.

Expected behavior
No errors in log

System Information:

• OS: Linux with Xorg
• GPU and GPU driver: AMD

F3D Information
Paste the content of f3d --version or f3d-console.exe --version on Windows: master

Additional context
Add any other context about the problem here.

[mwestphal]

FYI @noclone @snoyer @Meakk