Native bindings to the AMD_depth_clamp_separate extension.

The extension ARB_depth_clamp introduced the ability to control the clamping of the depth value for both the near and far plane. One limitation is that the control was for both planes at the same time; some applications can benefit from having clamping enabled for only one of the two planes, in order to save fillrate for the other plane by clipping the geometry.

This extension provides exactly such functionality.