Investigate using stenciled rendering planes to replace the out-of-view fragment discard technique.

[bryanedds]

We used to elide fragments of surfaces that are beyond the viewable range in the shader with GLSL discard, but that was waaaaay to inefficient (all uses of discard in shaders have since been removed). However, we might be able to efficiently restore this functionality with render stenciling, so we should investigate this.

Relevant TODO in code here -

Nu/Nu/Nu/Transform/Presence.fs

Lines 24 to 29 in 5bb97a3

	// TODO: see if we can use this to set up rendering stencil planes (presence would also have to become a key in batching if it isn't already).
	member this.DepthCutoff =
	match this with
	| Omnipresent -> Constants.Render.FarPlaneDistanceOmnipresent
	| Imposter -> -Constants.Render.NearPlaneDistanceImposter
	| Exterior | Interior -> Constants.Render.FarPlaneDistanceExterior