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.