This patch adds the -X option to wmsetbg to scale a background image across all heads.
wmsetbg-xinerama for Window Maker 0.92.0
Allows wmsetbg to revert to pre-0.90.0 behaviour when choosing a background image that is large enough to span several heads in a Xinerama setup. If the -X flag is passed to wmsetbg, the background image will be stretched to fill the logical screen (as it would be by default in older versions of wmsetbg) instead of the new behaviour of being tiled across screens. This option only has effect if wmsetbg is compiled with XINERAMA #defined.