Performance optimization of the FieldOfView script

[stilnat]

Summary

Checking the profiler, I noticed that the FieldOfView script is now taking a good chunk of the computation time per frame, it's in light blue on the picture below. That might be completely normal, but since it's taking around 15~20% of the CPU time on my computer, I think it's worth investigating.

Goal

Investigate the code of the FieldOfView script, try to find what's exactly require so much computation and if that could be reduced by a fair amount.

Context

This issue is from build 0.3.95h.