gdk: Make GDK_DEBUG GL backend selection global

gdk: Make GDK_DEBUG GL backend selection global

We have a global GdkGLBackendType now, just set it.

This way, using the variable forces the backend type, and we don't need
special code handling the env vars in the backends.

It also means setting the env var will now "work" on GDK backends that
don't even support that GL backend and simualte another GDK backend
having registered that GL backend already. So you can run
  GDK_DEBUG=gl-wgl gtk4-demo
on test what Wayland will do when WGL is in use.