GDK-Win32: Port to common EGL handling code

GDK-Win32: Port to common EGL handling code

This will port the EGL code in GDK-Win32 to use the common GDK code to
initialize EGL.  However, at the current state, although EGL is
correctly initialized, this code is disabled for now since
gdk_gl_context_make_current() fails as the shaders do not work for EGL
via libANGLE on Windows.

We can now clean things up in gdkglcontext-win32-egl.c as a result.