Add dispersion correction

[jgalazm]

From the paper "Tsunami propagation over varying water depths" of Taemin Ha and Yong-Sik Cho.

First implement the simpler formula this does not require any complex operations rather than summing stuff on the shader.

The other more complex formula is this one:

It is complex since it requires to get a,b at every point by fitting h = ax + by + c

[jgalazm]

That formula did not work because dx and dy need to be constant which requires dlat and dlon to be variable.

Better try it first in the shader with cartesian coordinates only.