The SVG icon was changed in df8e0785a3 but the small XPM icon
was not modified.
Unlike other XPM icons which are used in `ViewProviderSketch.cpp`,
this `PointOnPoint` XPM icon doesn't seem to be used at all.
Nevertheless, we still update it so that the XPM matches
its corresponding SVG.
Also update the `README.md` to explain how these XPM icons
are generated.
b46d08e748