Limit the zone where random points are drawn

[seal20]

Use case:
*The quadrat only occupied a small and distorded zone in the photos
*The user wants to draw random points in the quadarat zone only to avoid "off" points

Workaround cut the photos before analysis, if really distorted adjust the perspective using gimp or other similar software.

TODO:
*Define a polygon representing the quadrats
*Check if each x,y generated falls within this selection if not re-generate a new x,y if yes store the value.