SimGear Merge Request #98: LEFT_BIT and RIGHT_BIT node masks for VR visibility masks (merged)

This adds LEFT_BIT and RIGHT_BIT to the NodeMask enum in RenderConstants.hxx. These are used when setting the left and right cull masks so that the left one excludes RIGHT_BIT and vice versa.

A corresponding merge request for flightgear will pass these to osgXR so it can set them on visibility mask geodes in VR cameras it creates attached to the cameras, to overwrite the depth buffer to 0 on parts of the frame that won't be visible through the VR lenses.

[James Hogan]

@fgarlin