RotateTool centers on average instead of min/max

[m-7761]

Double-vertices, etc. throw off averages. There's lots of code that does center on min/max but it's all pretty funky. I've rewritten it somewhat. Probably would benefit from a utility function or class.

[m-7761]

The radius of the pivot should be a percentage of the selection's bounding box, instead of fixed, so it is visible at different scales/units.

It's also invisible on a green background, and not very visible on the teal background; so probably it should use the same XOR logic as the bounding box. Which shouldn't be white because that's invisible on gray. I've been using gray. It's less contrast but won't disappear into grays.

[m-7761]

FWIW there is some trickery if using XOR because the crosshairs formed by the planes of the rotate pivot draw directly on top of themselves, and so cancel each other out. The following depth-compression technique lets the depth-buffer prevent the canceling out.

            glEnable(GL_DEPTH_TEST);
                glDepthRange(0,0);
                glDepthFunc(GL_LESS);
                {
                    //glColor3f(0,1,0);
                    glEnable(GL_COLOR_LOGIC_OP);
                    glColor3ub(0x80,0x80,0x80);
                    glLogicOp(GL_XOR);
                    glTranslated(x,y,z);
                    rotatepoint_draw_manip(scale); //0.25f
                    glTranslated(-x,-y,-z);
                    glDisable(GL_COLOR_LOGIC_OP);
                }
                glDepthRange(0,1);
                glDepthFunc(GL_LEQUAL);

It looks good I think. Not everything can be XOR. Unfortunately.