Draw a grid on the wallpaper.

[Andrea Cimitan]

libgnome-desktop-3-2 should be patched to draw optionally a grid on the top of the wallpaper.
Attached an initial patch that applies the effect using cairo.

The patch is not complete, requires few more tunings:

1) Option (ideally, using gsettings), which enables/disables the pixel grid.
2) The filename used in local cache should be different when it has grid enabled or not.
3) The grid texture is currently stored at /usr/share/backgrounds/desktop_grid.png, an updated patch should use the global install dir plus a right prefix (or wallpaper_grid.png as image name too).