fix hidden cursor bug

On some systems loading a pixmap from an svg file can fail.  On these machines
an empty cursor results, meaning the cursor is invisible inside Rviz.  This
works around the problem by using an arrow cursor when the desired cursor
pixmap canot be loaded.

This is the same fix as https://github.com/ros-visualization/rviz/pull/778 (on hydro-devel) but applied to indigo-devel.