modernize depth sorting code

modernize depth sorting code

The following improvements were made to vtkDepthSortPolyData:

• transfrom GetCell to GetCellPoints. Building the cells one by one is expensive and we only need points to determine the depth. We can also then access the points in place.
• transform qsort to std::sort. Comparisons in std::sort get inlined so std::sort is much faster. I also reduced the overhead of swaps by using a functor so that only cell id is swapped.
• use templates to deal with point types instead of going through virtual GetValue API that converts them to double.
• restructure depth computations so that they can be vectorized by the compiler.
• I added a Cxx test to improve test coverage of the various sorting modes supported by this class.

The following improvements were made to vtkPolyData and vtkCellTypes:

• move the non-virtual impementations of GetCellPoints to the header to improve compiler optimization.
• add non-virtual GetCell method and also make it inline
• fix an issue in cell types where the MaxId is not set correctly when you pass in pre-built arrays

In tests the improved depth sort is ~ 3 x faster when sorting by first point and cell bounds, and ~ 2 x faster when sorting by parametric center. The tests made use of a ~ 8.1M cell iso-surface computed from a cosmology simulation, and gcc 4.9.2 compiled with -Wall -Wextra -DNDEBUG -Ofast -march=native -mavx -ffast-math

code	point	bounds	param
old	2.58 sec	3.05 sec	3.19 sec
new	0.82 sec	1.02 sec	1.65 sec
speed up	3.14 x	2.98 x	1.93 x