Visualization, TKOpenGl - handle flat shading model within GLSL programs

The following shading models are not supported by built-in GLSL programs (in contrast to deprecated FFP):
- V3d_COLOR
- V3d_FLAT

The first one is almost useless and can be safely removed (the same effect can be achieved by removing all non-ambient light sources).

The second one can be achieved by application by duplicating vertices for each triangle with unique normal, requires more memory and recomputation of presentation. Therefore, support from GLSL programs can be useful here.

Unfortunately, the behavior similar to V3d_FLAT is not trivial to achieve - it requires Geometry Shader (also fake flat model can be simulated by flat normal interpolation - which will give the single normal for entire triangle, but this normal will be taken from one of the vertices, not for triangle itself).

Anyway, current mechanism of global shading model should be also revised / completed with per-object options, since it is rarely desired to use only Flat or only Phong shading model for all objects in the scene. This, however, might be implemented in dedicated patch.