small bug fixes and new features (for "-mask-out", "-draw-spheres" and "DrawSpheres()")

Fixed several bugs in the "-mask-out", "-draw-spheres", "-draw-hollow-spheres" arguments for "filter_mrc". (I also probably fixed another bug that occurs when using "-mask" with tensor-voting, "-surface-tv".)

The capabilities of the "-draw-spheres" and "-draw-hollow-spheres" arguments, as well as the the "DrawSpheres()" function (in "draw.hpp") have been significantly augmented. (In addition to new features, the default behavior of these arguments and functions should be much more sensible now. Unfortunately, these changes break background compatibility.)