Native bindings to the NV_polygon_mode extension.

This extension adds a PolygonModeNV entry point which can be used to change the polygon rasterization method. Using this extension, state consistent with rendering triangle primitives can trivially be toggled to render primitives as lines or points. In addition, independent enables are provided for polygon offset in conjunction with these new point and line polygon modes.

This introduces a level of support for PolygonMode comparable with the OpenGL 4.3 core profile.