GDK-Win32: Realize EGL using common realization code

GDK-Win32: Realize EGL using common realization code

This will clean up the EGL code in GDK-Win32, as well as fixing crashes caused
by using an invalid EGL context in gdk_gl_context_make_current() as we did not
store up the EGL context in the correct place (lost during the transition to
the common EGL initialization code).

On the Windows/libANGLE side, the initialization of EGL has now fully moved to
the common code in GDK, but we will still default on WGL for now.  Help is
really appreciated for fixing the shaders on libANGLE!