This work follows Stefan Gustavson's paper "Simplex noise demystified"
http://staffwww.itn.liu.se/~stegu/simplexnoise/simplexnoise.pdf
without using uniform arrays or texture engines.

A single vec4 uniform 'pParam' is used to set the length and a and
c constants respectively of the permutation polynomial to use. The
last element of the vec4 is the number of gradients to select from
on the vertices. An example vec4 is ( 19*19,  2*19,  1,  7 )

Refer to http://en.wikipedia.org/wiki/Permutation_polynomial for
more information on permutation polynomials.