Add rounding functions for single-precision float and integer types

- Rounding an integer does nothing, but this allows one to use Pii::round()
  in template code for integer types without performance penalty.
Remove unreferenced variable in release mode
Fix color type precision in linear filter border handling
Improve Gaussian and Laplacian of Gaussian filters

- Changed Gaussian cutting from 3*std to 2*std
- Changed LoG scaling so that the minimum is always -4.