gdk/win32/gdkglcontext-win32.c: Fix think'o

Trying to test whether a GdkGLContext is a GLES context when it is NULL
is quite dumb, and we are actually asking for the assertion
that the GDKGLContext is NULL :| D'oh...

Instead, see whether we have set up an EGL display, which should always
be present when EGL is being used (and otherwise not when WGL is used),
and determine which way we should make no GL/EGL context current.
4 jobs for wip/master.win.egl in 30 minutes and 6 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Build
passed #259890
fedora-x86_64

00:05:57

passed #259891
win32
msys2-mingw32

00:06:39

 
  Flatpak
passed #259892
flatpak:demo

00:15:46

passed #259893
flatpak:widget-factory

00:16:08