GDK/Win32: Fix build after GDKGL refactoring

GDK/Win32: Fix build after GDKGL refactoring

There were some parts that need some updates after the refactoring in
GDKGL, so that the code will continue to build and run.

For gdkwindow-win32.c, comment out the parts where we check for use_gl
(which was removed), since we are going to move all drawing to OpenGL,
but don't remove/disable the whole portion as that transition is not
complete at this point.

There a is new GDKGL function that checks for the damaged area of the back
buffer, but since the notion of "damage" is for *NIX (GLX/EGL for
Wayland/mir), meaning that there is no such extension for Windows in this
regard, so we can't support this on Windows as-is, at least for now.

https://bugzilla.gnome.org/show_bug.cgi?id=773299