1. TGLParametricEquation. New class to define parametric surface. It can be
   defined by a) three equation in a "string form" (mathematical expression,
   the same as TF2) and as a pointer to function.
2. TGLParametricPlot. Painter to draw parametric surface. It has most of
   features other gl-painters have - box cut, zooming etc. + 21 color
   "schemes": you can change the color of surface (which is under cursor)
   by pressing 's' or 'S' - the same as TGLTF3Painter + wireframe mode ('w' or
   'W' key - temporary solution).


git-svn-id: http://root.cern.ch/svn/root/trunk@17525 27541ba8-7e3a-0410-8455-c3a389f83636