Use a lookup table to speed up the Lanczos kernel

Profiling the resize operation using Lanczos kernels showed that most of
the time was spent in the Sin function, which is called twice per
evaluation of the Lanczos kernel. Generating a lookup table and doing a
linear interpolation between table entries speeds up the resize by a
factor of 4.
Move splineKernel function nearer to Lanczos definitions.
Rename Lanczos functions using look-up tables to Lanczos2Lut, Lanczos3Lut and restore Lanczos2, Lanczos3 to give users the choice between higher speed or higher accuracy.
Update README.md
Fix a typo.
Add benchmarks for Lanczos3Lut to compare it with Lanczos3 performance.