GDK-Win32/4.0: Improve GL on Windows

GDK-Win32/4.0: Improve GL on Windows

Update the GDKGL implementation:

-Allow legacy contexts to be created.
-Use finer-grained attributes to ask for a pixel format when possible,
 which also adds support for anti-aliasing

In fact the changes here are required for GTKGL to work properly on
Windows for 4.x.

Note that creation of gles contexts is not done here, as the system does
not support such contexts directly on Windows, but only through means such
as ANGLE, which is a totally different issue here.

https://bugzilla.gnome.org/show_bug.cgi?id=773528