Use Int16Array instead of Object array for LUTs and uses positive indexes.

This results in 4x speedup of the storedPixelDataToCanvasImageData and
probably 12x speedups for storedColorPixelDataToCanvasImageData.
GenerateLut should also be much faster.
Merge pull request #4 from jpambrun/int16InsteadOfObjectArray

Use Int16Array instead of Object array for LUTs and uses positive indexes.