`Sector`: use point-in-polygon algorithm

[ForNeVeR]

See this interesting comment in the Sector code: https://github.com/ForNeVeR/bloodmasters/blob/9bec8244865c41cd18899fce577f33f4eafbcd6e/Source/Shared/Map/Sector.cs#L216-L225

Let's figure out if this is beneficial and implement if it is.

Look for // TODO[#114] in the game code.