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.
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