Native bindings to the NV_depth_clamp extension.

In some rendering applications such as shadow volumes, it is useful to allow line and polygon primitives to be rasterized without clipping the primitive to the near or far clip volume planes (side clip volume planes clip normally). Without the near and far clip planes, rasterization (pixel coverage determination) in X and Y can proceed normally if we ignore the near and far clip planes. The one major issue is that fragments of a primitive may extend beyond the conventional window space depth range for depth values (typically the range [0,1]). Rather than discarding fragments that defy the window space depth range (effectively what near and far plane clipping accomplish), the depth values can be clamped to the current depth range.

This extension provides exactly such functionality.