Polygon functions for usable sector shapes

[jmickle66666666]

https://github.com/sirjuddington/SLADE/blob/master/src/Utility/Polygon2D.cpp#L95-L127
https://github.com/jminor/omgifol/blob/master/demo/wad2obj.py

Being able to convert a sector to convex polygons is an incredibly useful feature. This can be used for many operations based around converting a map into other formats, or for creating 3d visualisations, increasing the applications of omgifol. Above linked are two possible sources to look at for implementation.