Blender Git Loki

Fix [#19997] Duplicating window results in graphics corruption in UI
reported by Micael Dias (and many others, see duplicates list)

On closing the first ("main") Blender window the very first OpenGL context
got deleted too. This context needs to be retained, since we share quite a
bit of OpenGL data through it to the newly created contexts (new windows).
Thanks to Ton Roosendaal for thinking out loud while trying to figure out
what the actual cause was.