Sign problem with horizontal displacement

[lecopivo]

There might be a problem with a sign in the horizontal displacement. As it is now, waves the round and not sharp.

I should investigate if the computation is really correct and potentially fix it.

The problem is probably around: https://github.com/lecopivo/WaterSurfaceWavelets/blob/69c88a77b1832b17f2e60e762128df465f7076ab/src/WaveGrid.cpp#L67