Add support for enabling only native windows

Add support for enabling only native windows

Some applications make weird assumtions on Gtk+ that do not work anymore
with the new client-side windows support. For instance SWT/Eclipse reorders
the stacking order of the X windows directly without telling gdk this,
which breaks gdk drawing as gdk now relies on knowing the stacking order
for window clipping.

This introduces a GDK_NATIVE_WINDOWS environment variable, which if set
causes Gtk+ to always use native windows. Its more compatible with
pre-csw Gtk+ behaviour if you do weird X-specific hacks, although it does
limit the size of GdkWindows to 65535x65535.