glDepthRange - specify mapping of depth values from
normalized device coordinates	to window coordinates

glDepthRange
	glDepthRange(zNear, zFar) -> None

void glDepthRange( GLclampd zNear,
                   GLclampd zFar )

zNear	 Specifies the mapping of the near clipping plane to
       window	coordinates.  The initial value	is 0.

zFar	 Specifies the mapping of the far clipping plane to
       window	coordinates.  The initial value	is 1.

After	clipping and division by w, depth coordinates range
from -1 to 1,	corresponding to the near and far clipping
planes.  glDepthRange	specifies a linear mapping of the
normalized depth coordinates in this range to	window depth
coordinates.	Regardless of the actual depth buffer
implementation, window coordinate depth values are treated
as though they range from 0 through 1	(like color
components).	Thus, the values accepted by glDepthRange are
both clamped to this range before they are accepted.

The setting of (0,1) maps the	near plane to 0	and the	far
plane	to 1.  With this mapping, the depth buffer range is
fully	utilized.

It is	not necessary that zNear be less than zFar.  Reverse
mappings such	as zNear=1, and	zFar=0 are acceptable.

GL_INVALID_OPERATION is generated if glDepthRange is
executed between the execution of glBegin and	the
corresponding	execution of glEnd.

glGet	with argument GL_DEPTH_RANGE

glDepthFunc, glPolygonOffset,	glViewport