Recast part 2: PLC_Navigation

Adds a new PolyListContext value that suggests we want polygon data related to navigation. This is used by a couple of unfriendly classes, GroundPlane and TerrainBlock, to ensure that the data they pass back is actually inside the request box. Previously, both classes would give back their whole collision data in response to even a small local request.

I've decided to submit three pull requests for recast since there are three distinct sets of changes to be made.
 1. Adding Recast library code
 2. This request, making changes to the Torque engine to accommodate getting polys for Recast
 3. Actual classes that wrap and make use of Recast/Detour
I figured it would be easier to break these up - especially number 2, since people will probably have differing opinions on my changes.