Add an "object cornerTri" for level construction

[iksbob]

... consisting of an equilateral triangle with accurate collision detection. The object should have location and extents values similar to a ramp. The extents values should scale a default model which fills a 1x1x1 cube, where the edges of the triangle are the corner-to-corner bisector lines of three of the cube's sides. See https://www.youtube.com/watch?v=rmP3UIH28J0 for illustration of the geometry.