GL_ARB_polygon_offset_clamp
http://www.opengl.org/registry/specs/ARB/polygon_offset_clamp.txt
GL_ARB_polygon_offset_clamp

	GL_POLYGON_OFFSET_CLAMP 0x8E1B
	void glPolygonOffsetClamp (GLfloat factor, GLfloat units, GLfloat clamp)