Rendering improvements: better caching of tessellations.

[vibraphone]

The current caching mechanism for vtkModelMultiBlockSource requires every polydata to be rebuilt, even when this is not necessary. Instead, only new and modified entities should be retessellated. This will be tricky because some modeling kernels (discrete, cgm) reuse the same entity handle for some operations (e.g. the Translate operator, the workpiece bodies in the boolean operators, etc.). In this case, we'll probably have to save a generation number with each PolyData and find some way to force changes on select entities of a model.

[vibraphone]

Methods in the model Manager that set the tessellation now add/increment a generation number. See SMTK_TESS_GEN_PROP (and the matching property for analysis meshes).