parameterize max shadow length

Currently has a hardcoded max shadow length of 600 cells (ie, it looks back towards the light source for an obscuring pixel a maximum of 600 cells). 

This should be parameterized, enabling different speed and appearance tests. For example, 600 works on 10m DEMs (which was used for the bulk of testing) but may be too aggressive (cast too long of shadows) with a 15 degree sun on a 100m DEM.